Preheat oven to 350. Spray a 9x9 baking pan with cooking spray.
4 pints fresh blueberries, rinsed and dried
3/4 cup sugar
4 t. cornstarch
1/2 t. lemon zest
Toss above ingredients in a bowl, and set aside.
1 1/4 c. flour
1 c. brown sugar
1/2 cup oatmeal
1 1/2 cubes unsalted butter, cut into small pieces
Combine flour, brown sugar and oatmeal in a food processor. Add cold, cut butter, and process briefly until blended, but still lumpy.Spread berry mixture into prepared baking dish. Spread crumble mixture on top. Bake at 350 for 60 minutes. Serve warm with vanilla ice cream if desired.