Preheat oven to 350°F. Combine rhubarb, strawberries, 3/4 cup sugar, and cornstarch or tapioca. If using tapioca, let the filling sit for 15 minutes for the tapioca to absorb some of the juices of the fruit.
4 cups chopped rhubarb, 2 cups chopped strawberries, 3/4 cup granulated sugar, 3 Tablespoons cornstarch
Add the fruit mixture and any accumulated juices to a 9x13-inch baking dish.
Combine flour, 1 cup sugar, baking powder, and salt in a large bowl. Add cold, cubed butter and use a pastry cutter to cut the butter into the dry ingredients until it resembles coarse crumbs about the size of peas.
2 cups all-purpose flour, 1 cup granulated sugar, 2 teaspoons baking powder, 1/4 teaspoon salt, 3/4 cup cold salted butter
Add the hot water and vanilla extract to the cobbler mixture, stirring just until it starts coming together. It's okay if some of the flour mixture is still shaggy and not completely incorporated.
1 teaspoon vanilla extract, 1/2 cup boiling hot water
Sprinkle cobbler topping mixture over the fruit filling in the pan, spreading it out evenly to cover the top. It's okay if there are some gaps where the fruit is showing. Sprinkle with 2-3 teaspoons additional granulated sugar for a crunchy sweet finish, if desired.
Bake for 40-45 minutes until golden brown on top and the fruit filling is bubbling around the edges. Cool for at least 15 minutes before enjoying with a scoop of ice cream.