Prepare corn bread mix according to package directions, using 2/3 cup fat-free milk and 2 egg whites; cool completely. Crumble to measure 3 cups; reserve remaining corn bread for another use.
Reduce oven temperature to 375°.
Place sausage in a food processor, and pulse until finely chopped. Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add the onion and mushroom caps to pan; sauté 5 minutes or until tender. Stir in sausage and spinach; sauté 2 minutes.
Drain oysters, reserving 1/3 cup oyster liquor. Combine oysters, reserved oyster liquor, corn bread, spinach mixture, French bread, and remaining ingredients in a large bowl; toss well. Place mixture in a 13 x 9-inch baking dish coated with cooking spray. Bake at 375° for 35 minutes or until lightly browned.