Preheat your oven to 350°F (175°C).

Prepare the fruit filling by mixing together chopped strawberries, rhubarb, brown sugar, cornstarch, and lemon juice in a large mixing bowl.

Combine the crumble topping ingredients by mixing flour, oats, brown sugar, and cold butter in a separate bowl.

Press half of the crumble mixture firmly into a greased baking dish.

Spread the prepared fruit filling evenly over the pressed base.

Sprinkle the remaining crumble topping over the fruit filling.

Bake in the preheated oven for 40-45 minutes or until the topping is golden brown and the filling is bubbly.

Cool the bars completely on a wire rack before slicing them into squares.