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

In a large mixing bowl, combine chopped strawberries, rhubarb, sugar, cornstarch, and lemon juice. Stir gently and let sit for 10-15 minutes.

In another bowl, whisk together flour, oats, brown sugar, cinnamon, baking powder, and salt. Mix in the cold cubed butter until the mixture resembles coarse crumbs.

Press about two-thirds of the crumb mixture into the bottom of the baking pan, then evenly spread the fruit filling over it.

Sprinkle the remaining crumb mixture on top of the fruit filling and bake for 40-45 minutes until the top is golden brown.

Remove from oven, let cool completely on a wire rack, then refrigerate for about an hour before slicing into squares.