This makes enough to feed a crowd. For a smaller group, halve the recipe and make it in an 8x8-inch pan. Prepare to eat it all within a few days as the moisture from the fruit tends to uncrisp the topping over time.
21/2 cups all-purpose flour, divided
1 cup light brown sugar, firmly packed
1/2 tsp. baking powder
1/2 tsp. salt
12 Tbsp. unsalted butter, cold, cut into 6 pieces
2 oz. cream cheese
1 large egg
1/2 tsp. vanilla extract
1/2 cup chopped walnuts (optional)
1/4 tsp. ground cardamom (optional)
1/2 pound (2 cups) strawberries, hulled, halved, and thinly sliced
1 pound (4 cups) rhubarb, cut into roughly 1/2-inch thick pieces
1/2 cup light brown sugar, firmly packed
1 tablespoon lemon juice
Preheat the oven to 350°F. Grease a 9x13-inch pan.
For the dough, put 2 cups of flour into the bowl of the food processor along with the brown sugar, baking powder, and salt. Add the butter and cream cheese, and pulse the motor until the mixture is moist and crumbly. Dump everything out into a medium bowl. Measure 3 cups of the mixture and add back to the food processor along with the remaining 1/2 cup of flour, egg, and vanilla. Blend just until it starts to come together into a dough. Press the clumps evenly into the pan. Bake for 10 minutes until puffy.
Meanwhile, combine the strawberries, rhubarb, brown sugar, and lemon juice in a large bowl. Spread the fruit in an even layer over the surface of the partially baked crust.
To the bowl of reserved streusel topping, add the walnuts and cardamom if desired, and work in with your fingers until pebbly. Sprinkle the topping over the surface of the fruit. Bake again for 40 to 50 minutes until the topping is light golden brown. Let cool completely before slicing. Store uncovered for a day or two to keep the streusel from getting soggy.