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.
In a medium bowl: add chopped strawberries, cornstarch, sugar and lemon juice. Mix everything together and set aside until ready to use.
In a bowl: combine flour, oats, brown sugar, baking powder, salt and mix. Add melted butter and mix everything together. Reserve 1 cup of oatmeal mixture & set aside.
Add the rest of the oatmeal mixture to the pan and press to the bottom to form the crust (use a measuring cup to press down). Spread and level out the strawberries filling on top of the crust. Sprinkle the reservedoatmeal mixture (1 cup) on top filling. Gently press the oatmeal down to the strawberries.
Bake for 45-50 minutes or until the crumble is lightly golden brown.
Remove the pan from the oven and let oatmeal bars cool completely for at least 2 hours in the pan. Cut into 9 pieces.
Refrigerate the leftovers in an airtight container.