Preheat the oven to 350F. Line an 8x8 or 9x9 baking pan with parchment. Lightly spray with cooking spray.
In a large mixing bowl, combine the oats, gluten-free flour, brown sugar, baking powder, salt and butter with a fork into a crumble-like mixture.
Press ⅔ of the mixture into the bottom of the prepared baking pan. Bake for 10 minutes.
Place raspberries in a medium, microwave-safe bowl. Microwave until just thawed, 2-3 minutes, stirring halfway through.
Drain any excess liquid off the raspberries and return to the bowl.
Add the sugar, gluten-free flour, cornstarch, and lemon juice and mix to combine.
Pour the raspberry mixture on top of the baked bottom crust. Sprinkle with remaining crumble.
Bake for another 25-30 minutes, until golden and bubbly.
Let the bars cool completely before slicing and serving. Enjoy!
