In a medium saucepan combine blueberries, the 2 cups strawberries, the granulated sugar, and the 2 tablespoons flour. Cook and stir over medium heat until mixture is thickened and bubbly. Transfer fruit mixture to an 8x8x2-inch metal or disposable foil baking pan; set aside.
For topping, in a medium bowl combine oats, brown sugar, the 1/4 cup flour, the nutmeg, and cinnamon. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Sprinkle topping over fruit mixture. Cover pan tightly with foil.
For a charcoal grill, grill dessert in pan on the rack of an uncovered grill directly over medium coals for 20 to 25 minutes or until topping is set. (For a gas grill, preheat grill. Reduce heat to medium. Place dessert in pan on grill rack over heat. Cover and grill as above.)
Serve warm. If desired, serve with whipped cream and additional strawberries.