Pirogy Casserole

1 pound extra wide noodles, cooked
16 ounces Cheese Whiz
8 ounces Colby cheese, grated
1 cup minus 2 tablespoons butter or margarine
1 or 2 large onions, sliced
Enough prepared instant mashed potatoes
    for 12 servings (or use leftover potatoes)

Topping:
1/2 cup Parmesan cheese
Parsley flakes

Saut onion and butter. Add Cheese Whiz. Stir until melted and combined. Put a small amount of this mixture into bottom of a 13 x 9-inch baking pan. Put half of the noodles on top; then half the potatoes, in small mounds. Top with half of the grated Colby, then half of the melted cheese/onion mixture. Repeat layers. Top with Parmesan cheese and sprinkle with parsley flakes. Bake 350 degrees F for 50 minutes or until bubbly.