Strawberry Rhubarb Filling
2 ½ cups strawberries chopped
2 ½ cups rhubarb chopped
1 Tablespoon lemon juice
1 cup granulated sugar
2 Tablespoons cornstarch
Biscuit Topping
1 ½ cups all purpose flour
¼ cup sugar plus extra for topping
2 Tablespoons baking powder
¼ teaspoon kosher salt
6 Tablespoons unsalted butter cold and cut into small pieces
⅔ cup whole milk plus extra for topping
½ teaspoons vanilla extract