Adapted from Dorie Greenspan's recipe
Center a rack in the oven and preheat oven to 350 F.
Butter a 9-inch square baking pan (I used one 7x10 inch and one 5x7 inch baker).
Line a baking sheet with a Silpat or piece of parchment paper.
Step 1: Crisp Crust and Topping
1 cup (5 oz.) all-purpose flour
1 cup (7 oz.) packed light brown sugar
3/4 cup old-fashioned rolled oats
1/2 tsp. ground cinnamon
1/2 cup finely chopped walnuts
1 stick (8 Tablespoons) unsalted butter, cut into pieces and chilled
Put the flour, brown sugar, oats, cinnamon, and salt in the bowl of a stand mixer and mix on low speed until combined. Add the butter a few pieces at a time on medium speed until butter is mixed in and the mixture looks like wet sand (this can also been done by hand with a pastry blender). Add in the chopped nuts.
Spoon half of the mixture into the baking pan and pat it down to form a thick crust; set aside the remainder for the topping.
Step 2: For the Filling
For the filling:
12 ounces (about 3 cups) strawberries, hulled and sliced
1 orange, zested and juiced
2 Tbsp. balsamic vinegar
1/4 cup cornstarch
1 cup (7 ounces) sugar
1 tsp. vanilla extract
1 pound (about 4 or 5 stalks) rhubarb, trimmed and peeled if skins are tough, sliced into 1/2 inch wide pieces.
Put orange juice and balsamic vinegar in a 1-cup measuring cup. Add cold water to make 1/2 cup total, mix in cornstarch until dissolved, set aside.
Put the strawberries in a medium saucepan add orange zest and sugar. Place the pan on medium heat and crush the berries with a potato masher as pan heats. Stirring occassionally, bring the mixture to a full boil. Pour the dissolved cornstarch mixture into the pan, and stirring with a whisk, bring everything back to a boil. Keep cooking and stirring until the strawberry filling is thick and no longer cloudy, about 3 minutes. Pull the pan from the heat, stir in the vanilla and set aside.
Step 3: Assembly and Baking
Pour the warm strawberry filling over the rhubarb.
Spread the remaining crisp mixture evenly over the filling, breaking it up with your fingers as you go.
Place baking pan on baking sheet, bake about 60 minutes, or until the topping is golden and the strawberry jam is bubbling up all around the edges. Transfer the pan to a rack to cool until just warm or at room temperature.
Step 4: Enjoy!
Tastes best the day it's made, crust gets a bit soggy but can still be enjoyed the next day, warm it up a bit in the microwave.