Spray a 9 x 13 inch pan with non-stick cooking spray. If using rice, pasta or boiled potato stir 1 cup of sauce into the rice, pasta or boiled potato. If using mashed potatoes, you may press them into the bottom of the pan to form a crust, or you may use them as the topping of your casserole. Combine the remaining 3 cups of sauce with the poultry/fish and the vegetables. Pour the mixture over the ingredients in the pan. Toss together the topping ingredients (butter, breadcrumbs and cheese). Sprinkle evening over the top of the casserole. Cover and refrigerate for up to 2 days or freeze for up to six weeks. Defrost the casserole overnight in the fridge if necessary. Allow the casserole to come to room temperature for about 30 minutes. Preheat oven to 350oF. Bake the casserole for 30 minutes or until it is bubbling and top is golden. Serve immediately.