• Completely thaw 6 cups frozen mixed berries. Drain any excess liquid.
• Preheat oven to 400 degrees.In a large mixing bowl, mix berries, 1 cup sugar, ¼ cup cornstarch, 2 tbs lemon juice and 1 teaspoon vanilla. Toss until evenly coated. 
• Pour berry filling into a 9 inch pie crust. Pour berry mixture into a 9 inch deep dish pie crust.

Create the Oatmeal Crumble Topping

• Cut 9 tbs of salted butter into slices.
• In a seperate mixing bowl, combine 1 cup old-fashioned oats, ¾ cup brown sugar, 1 teaspoon cinnamon, ½ cup all purpose flour, and butter slices. Mix with a pastry blender until pea sized clumps form.
• Sprinkle oatmeal crumble topping evenly over the mixed berries until completely covered.
• Bake for 30 minutes.
• Reduce oven temperature to 350 and bake an additional 15-20 minutes.
• Cool for 2 hours at room temperature before serving.