Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish.

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

In a separate bowl, mix together flour, oats, brown sugar, cinnamon, baking powder, and salt. Cut in the cold butter until the mixture resembles coarse crumbs.

Press two-thirds of the crumb mixture into the bottom of the prepared baking dish, then pour the fruit filling over it.

Sprinkle the remaining crumb mixture over the fruit filling. Bake for 40-45 minutes until the top is golden brown.

Let the bars cool completely in the pan on a wire rack before slicing.