4 cups blueberry, divided
3/4 cup sugar
2 tablespoons cornstarch
1 cup flour
3/4 cup quick-cooking oats
1 cup packed brown sugar
1/2 cup cold margarine

Preheat oven to 350 degrees. Spray an 8-inch pan with cooking spray.

Crush one cup of berries and add enough water to bring level back up to one cup. Combine crushed berries, sugar and cornstarch in a small saucepan and bring to a boil. Cook and stir until thickened, approximately two minutes. Remove from heat. Stir in remaining 3 cups of blueberries and set aside to cool.