Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking dish by greasing it with butter or lining with parchment paper.

In a medium bowl, combine chopped strawberries, rhubarb, sugar, cornstarch, and lemon juice. Stir gently and let sit for about 10 minutes.

In a large mixing bowl, whisk together all-purpose flour, oats, brown sugar, cinnamon, baking powder, and salt. Cut in cold butter until resembling coarse crumbs.

Press about two-thirds of the crumb mixture into the bottom of the prepared baking dish. Pour the fruit filling over the crust.

Sprinkle the remaining crumb mixture evenly over the fruit filling. Bake for 40 to 45 minutes until the topping is golden brown and filling is bubbly.

Allow cooling in the pan on a wire rack for at least an hour before slicing into squares.