Prepare the filling by washing and hulling the strawberries, then chop them. Chop the rhubarb into similar-sized pieces. Combine strawberries, rhubarb, granulated sugar, cornstarch, vanilla extract, and lemon juice in a bowl and set aside.
Make the crumble topping by mixing flour, oats, brown sugar, baking powder, and salt in a bowl. Add melted butter and mix until crumbly.
Press two-thirds of the crumble mixture into a greased 9x13-inch baking dish. Spread the fruit filling evenly over the crust and sprinkle the remaining crumble on top.
Bake in a preheated oven at 350°F for 35-40 minutes until golden brown and bubbling. Allow to cool completely before cutting into squares.