In a medium bowl: add raspberries, cornstarch, sugar and lemon juice. Use a fork and mash the raspberries. Set aside until ready to use.
Crumble Layer
In a bowl: combine flour, oats, brown sugar, baking powder, salt and COLD butter. Use a pastry cutter or your hand to mix until it begins to form coarse crumbs.
Bake
Preheat the oven to 350°F. Place parchment paper with overhanging edges on the bottom of the 9"x 9" pan (this will help when removing the crumble bar from the pan later). Grease the sides with cooking flour spray or butter.
Press ⅔ of the crumble mixture into the bottom of the pan (use a measuring cup to press down). Bake in the oven for 10 minutes.
Remove the pan from the oven. Spread and level out the raspberries filling on top of the crust. Sprinkle the rest of the crumble mixture on top filling. Return the pan to the oven and bake or 40 minutes or until the crumble is lightly golden brown.
Remove the pan and let cool completely for at least 2 hours, or chill in the refrigerator overnight. For best results, let the bar firm up before cutting. Remove the bar from the pan. Cut into 9 pieces.