This yummy simple triple berry rhubarb slab pie – a pastry pie in bar form – is baked up with 3 berries and rhubarb for all the berry/rhubarb lovers out there.
I am so excited to share with you this Triple Berry Rhubarb Slab Pie. I am excited to share it with you because it is deliciously sweet from blueberries, raspberries, blackberries, and my homegrown rhubarb. But I am almost as excited to share it with you because I FINALLY made a wonderful, homemade flaky pie crust. I avoid making pie crusts, basically ALWAYS. I will bake an occasional pie but purchase the crust, usually using a refrigerated premade crust. Since making this recipe and following the instructions to a T, (is it T or t or tee?) I can claim that I can make pie crust. I am even willing to make pies again and again. Just look at that beautiful, tender pie crust.
What is a slab pie. A slab pie is a pastry baked on a jelly roll pan, assembled like a pie, but served liked a bar cookie. It is super easy, fairly quick – minus the dough cooling time, and bakes up delicious.
WHAT DO I NEED TO MAKE A TRIPLE BERRY RHUBARB SLAB PIE?
FOR THE FILLING –
Three different kinds of berries – such as blueberries, raspberries, blackberries, or even strawberries (I used prepackaged frozen berries that I set out to thaw for about 30 minutes)
Rhubarb – frozen or fresh, but diced small about the size of the blueberries
Cornstarch – don’t forget the cornstarch because it is the thickener
Lemon juice – the acid in the lemon juice balances the sweetness of the fruit and brings out more of the natural flavor of the fruit
FOR THE CRUST –
All purpose flour
Coarse sugar – for a decorative sprinkling on top of the crust before baking
A little salt
One egg – yolk and egg whites divided
Cold butter – but slightly softened (I took the butter out of the fridge and microwaved it for about 15 seconds)
Half & half – half & half is thicker than whole milk but you can use whole milk, you may need to use a little more or a little less milk
FOR THE ICING –
Powdered sugar – shifted to remove any clumps
Almond extract – the almond flavoring is a nice accomplainment to the berries and rhubarb, but if preferred you can use only vanilla extract
HOW DO I MAKE A TRIPLE BERRY RHUBARB SLAB PIE?
FOR THE CRUST –
Dice the butter into small pieces and cut into the flour and salt with either a pastry cutter or fork. I usually use a fork because I feel like I have better control of the butter. Continue to cut the butter in until it is like a small pea size and mixed throughout. Whisk the egg yolk in with the half & half, and then add 1 Tablespoon at a time, using the fork to mix it in. Only add enough milk to allow for the pastry dough to form into a ball. Once this is achieved, stop. The less handling the pastry dough the better for a tender crust. Divide the ball into 2 smaller balls, wrap in plastic wrap, place in fridge for up to 1 hour.
After 1 hour, remove 1 ball of pastry dough, roll out to a 1/4 inch thickness and as close to 11×8 as possible. Again, try to handle the dough as little as possible. Place the dough in the jelly pan.
For the top of the slab pie, roll out the 2nd ball of pastry dough the same as the first. You can either top the triple berry rhubarb slab pie with a whole crust or form a decorative crust to go on top. Forming a decorative crust is easier than you think. Using a butter knife cut 1/2 inch strips of pastry dough and lay diagonal across the berries, spacing every 1/2 inch. Or you can even cut the 1/2 inch strips in half lengthwise, leaving 1/2 inch whole at the top; braid the 2 pieces together and lay diagonal or straight across the filling.
Make an egg wash with the egg whites and a little water or milk, brush this over the top layer or strips. Sprinkle a little coarse sugar over this. This adds a slight shine, a light browning, and a delicious crunch to the top crust I think you will really enjoy.
FOR THE BERRY RHUBARB FILLING –
While the pastry dough is cooling in the fridge, start to prepare the triple berry rhubarb filling. Wash and dice the rhubarb small, about the size of blueberries; measure and place in a large bowl. Measure and add the frozen berries, mix together with the rhubarb. Stir in the cornstarch, sugar, and lemon juice. Let this sit while you prepare the bottom crust and top crust. I have only made this with these slightly thawed mixed berries and it was just right. I can’t confirm how thick or runny the filling will be if you use fresh berries.
Once the first pie layer is set on the baking sheet, spread the berry rhubarb filling all over the bottom crust. Top the filling with the second pastry crust, following the above suggestions. If you choose to lay one whole crust on top instead of the strips, pierce the top with a fork every few inches. You can pierce in a decorative pattern if desired.
Bake the triple berry rhubarb slab pie for 35 to 40 minutes in a preheated 375 degree oven.
FOR THE ICING –
Shift the powdered sugar into a medium sized bowl. Whisk or stir in the milk, 1 Tablespoon at a time, until the glaze is the consistency you desire (I like it just so I can drizzle the icing over the top of the slab pie.) Stir in the vanilla and almond extracts. After the triple berry rhubarb slab pie has cooled for at least 10 minutes, drizzle the icing over the slab pie. Drizzle as much or as little as you desire.
Let the slab pie continue to cool before serving for ultimate thickening. If served warmed, which is delicious, the slab pie might be a little runny. Serve as is, with a scoop of ice cream, or whipped cream. Store any remaining triple berry rhubarb slab pie covered in the fridge.
Triple Berry Rhubarb Slab Pie
Slab pie pastry
- 1 5/8 cup all purpose flour
- 1/2 cup butter (slightly soften, cut into small pieces)
- 1/2 tsp salt
- 3 oz half and half (plus a little extra as needed)
- 1 egg (divided by egg yolk and egg white)
- 1/4 cup coarse sugar
Triple Berry Rhubarb Filling
- 2 cups rhubarb (diced)
- 2 cups frozen triple berry (blackberries, raspberries, and blueberries(slightly thawed))
- 1 cup sugar
- 3 Tbs corn starch
- juice from half a lemon
- 2 cups powdered sugar
- 2 to 3 Tbs milk
- 1/2 tsp vanilla extract
- 1 tsp almond extract
- In a large bowl, combine flour and salt
- Cut in the butter with a fork until it is crumbly
- Whisk the half & half and yolk together
- One Tbs at a time gradually add the half & half and yolk to the flour mixture tossing it with your fork until if forms a ball (do this ever so gentle)
- Add a little more half & half as needed to form a ball
- Divide your dough in half and wrap each half in plastic wrap
- Refrigerate for 1 hour to chill dough to make it easier to roll out, remove from fridge
- Preheat your oven to 375 degrees
- Place wax paper down on your counter with a slightly damp dishrag underneath (this holds your paper in place), sprinkle with flour
- Roll, using a rolling pin, one pastry ball to 1/4 nch thickness and to fit your 11 x 8 jelly roll pan
- Wrap the dough around the pin gently to lift onto the jelly roll pan and unroll the dough. Stretch your dough gently to the edges and up the sides of your pan as needed
- Spoon and spread your berry/rhubarb mixture over the top of the pastry dough
- Roll the second ball to a thickness of 1/4 inch and approximately 11 x 8 inches
- Cut with a butter knife strips about 1/2 inch wide, carefully lift and place diagonally on top of berry/rhubarb mixture. Continue until strips are all across slab pie with about 1/2 inch inbetween strips.
- Make an egg wash with leftover egg white and a little milk or water, brush over the pastry strips
- Sprinkle strips with coarse sugar
- Bake at 375 degrees for 35 to 40 minutes until your slab pie is browned and finished baking
Triple Berry Rhubarb Filling
- While the pastry is cooling in the fridge, in another large bowl put your diced rhubarb and frozen berries, mix together
- Sprinkle your corn starch over the top of the berry mixture and stir in
- Squeeze your lemon juice over the berry mixture and stir in
- Add your sugar to the berry mixture and stir in
- In a small bowl add your powdered sugar
- Adding 1 TBS at a time, add your milk, stirring in between until it is glazing consistency
- Add your vanilla and almond extract, stirring in
- When your slab pie has cooled for about 10 minutes, glaze with a spoon by drizzling back and forth your icing
- 11x8 baking sheet
Enjoy this delicious triple berry rhubard slab pie as a summery dessert!
Adapted from and inspired by Taste of Home Raspberry-Rhubarb Slab Pie
Triple Berry Rhubarb Slab Pie was orginally posted June 12, 2014; images and content were updated June 2021 for clarification.