Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish or line it with parchment paper.

Combine chopped strawberries, rhubarb, sugar, cornstarch, and lemon juice in a medium bowl. Toss gently and let sit for 10 minutes.

In a large bowl, whisk together flour, oats, brown sugar, cinnamon, baking powder, and salt. Cut in butter until mixture resembles coarse crumbs.

Press 2/3 of the crumb mixture into the bottom of the prepared baking dish. Spread fruit filling evenly over the crust.

Sprinkle remaining crumb topping over the fruit filling. Bake for 40-45 minutes until topping is golden and filling is bubbling.

Remove from oven and cool completely in the pan before cutting into squares.