You can assemble this entire dish up to a week ahead. Bake the corn muffin mix ahead; store in an airtight container in the freezer for a couple of days. Assemble the casserole according to directions; cover and freeze. A day in advance, take it out of the freezer and thaw in the refrigerator (about 24 hours). Uncover and let stand 30 minutes at room temperature before cooking; bake as directed.
Prepare corn muffin mix according to package directions; cool. Crumble muffins into a large bowl; stir in bread. Set aside.
Remove casings from sausage. Cook sausage and onion in a large nonstick skillet over medium heat until browned, stirring to crumble. Drain.
Combine milk, cumin, pepper, tomatoes, and egg substitute; stir with a whisk until well-blended. Add sausage mixture; stir well. Stir into bread mixture. Spoon half of bread mixture into an 11 x 7-inch baking dish coated with cooking spray. Top with 1/2 cup cheese. Spoon remaining bread mixture over cheese. Cover and refrigerate 8 hours or overnight.
Preheat oven to 350°.
Bake casserole at 350° for 20 minutes or until set. Top with 1/2 cup cheese, and bake an additional 20 minutes or until set. Let stand 10 minutes before serving.