Preheat oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.

Prepare the crumb mixture: In a large bowl, whisk together the flour, rolled oats, brown sugar, baking powder, and salt.

Cut in the cold, cubed butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces remaining. Measure out 1.5 cups of this mixture and set aside for the topping.

Press the remaining crumb mixture evenly into the bottom of the prepared baking pan to form the base. Bake the crust for 12 minutes until lightly golden. Remove from the oven.

Prepare the filling: In a medium bowl, gently toss the chopped rhubarb, strawberries, granulated sugar, cornstarch, and vanilla extract until the fruit is evenly coated. Spread the fruit mixture evenly over the pre-baked crust.

Sprinkle the reserved 1.5 cups of crumb topping evenly over the fruit layer.

Bake for an additional 30–35 minutes, or until the topping is golden brown and the filling is bubbly around the edges. Let the bars cool completely in the pan on a wire rack before lifting out by the parchment overhang and cutting into 12 bars.