Melt corn oil margarine in a large skillet.

Sauté sliced mushrooms and thinly sliced onion in the skillet for 6 to 8 minutes, until tender.

Season with pepper to taste.

Remove the mushroom and onion mixture from the pan and set aside.

In the same pan, simmer mild Italian sausage in 1/2 cup of water, covered, for 10 to 12 minutes.

Drain the water from the pan.

Slice the sausage into 1/4-inch slices.

Brown the sausage slices slightly in the pan.

Drain any excess fat from the sausage and place the slices on paper towels.

Spray a 13 x 9-inch (3-quart) baking dish with nonstick vegetable spray.

Layer half of the wheat bread slices in the baking dish.

Spread half of the mushroom mixture over the bread.

Distribute half of the sausage slices over the mushroom mixture.

Sprinkle half of the grated reduced-fat Cheddar cheese over the sausage.

Repeat the layers with the remaining bread, mushroom mixture, sausage, and cheese, ending with cheese on top.

In a separate bowl, whisk together the liquid egg substitute, skim milk, buttermilk, Dijon mustard, dry mustard, nutmeg, and salt.

Pour the egg mixture evenly over the layered ingredients in the baking dish.

Sprinkle chopped fresh parsley and paprika over the top.

Bake in a preheated oven at 350°F (175°C) for 30-40 minutes, or until the casserole is set and the cheese is melted and bubbly.