01 -
Preheat oven to 350°F. Combine rhubarb, strawberries, 3/4 cup sugar, and cornstarch or tapioca in a bowl. If using tapioca, allow the filling to rest for 15 minutes to absorb fruit juices.
02 -
Transfer the fruit mixture and any accumulated juices to a 9×13-inch baking dish.
03 -
In a large bowl, whisk together flour, 1 cup sugar, baking powder, and salt. Add cold, cubed butter and use a pastry cutter to incorporate until the mixture resembles coarse pea-sized crumbs.
04 -
Pour hot water and vanilla extract into the flour mixture, stirring just until the dough begins to come together. Some dry, shaggy bits remaining is perfectly acceptable.
05 -
Scatter the topping mixture evenly over the fruit filling. Small gaps exposing the fruit are fine. For extra sweetness and texture, sprinkle 2-3 teaspoons additional granulated sugar over the top if desired.
06 -
Bake for 40-45 minutes until the top is golden brown and fruit filling bubbles at the edges. Allow to cool for at least 15 minutes before serving with ice cream.