1. Place frozen sausage in a large skillet coated with cooking spray; cook over medium heat until lightly browned on all sides and thawed, stirring frequently (about 8 minutes). Remove from pan to cool. Cut sausages into 1/2-inch slices.
2. Heat butter in pan over medium heat. Add mushrooms, and sauté 4 minutes. Add onions; sauté 2 minutes. Remove from heat; stir in tomato and 1/4 teaspoon salt.
3. Coat an 11 x 7-inch baking dish with cooking spray, and place half of bread cubes in bottom of dish. Top with half of sausage, half of vegetable mixture, and half of cheese. Repeat layers with remaining bread cubes, sausage, vegetable mixture, and cheese.
4. Combine egg substitute, eggs, milk, mustard, and remaining 1/4 teaspoon salt in a bowl, stirring with a whisk. Pour mixture evenly over casserole. Cover and refrigerate at least 8 hours.
5. Let casserole stand at room temperature 30 minutes before baking.
6. Preheat oven to 350°.
7. Bake at 350° for 45 to 50 minutes or until center is set. Let stand 10 minutes.