In a medium saucepan, combine the blueberries, rhubarb, granulated sugar, and the 2 tablespoons flour. Cook and stir until thickened and bubbly. Transfer fruit mixture to an 8x8x2-inch metal baking or foil pan; set aside.
For topping, combine oats, brown sugar, the 1/4 cup flour, nutmeg, and cinnamon in a medium bowl. Cut in butter or margarine until mixture resembles coarse crumbs. Sprinkle topping over fruit mixture. Cover pan tightly with foil.
Grill for 20 to 25 minutes or until topping is set. Serve warm with whipped cream, if desired. Makes 6 servings.