1. Combine mustard and mayonnaise in a small bowl. Spread 3/4 teaspoon mustard mixture over each of 4 bread slices; layer each slice with 1 1/2 ounces ham and 1/4 cup cheese. Sprinkle evenly with pepper. Spread 3/4 teaspoon mayonnaise mixture over each remaining bread slice; place, mustard side down, on top of sandwiches.
2. Combine egg substitute and fat-free milk in a shallow dish. Dip both sides of each sandwich into the egg mixture.
3. Heat a large nonstick griddle or skillet over medium heat. Coat pan with cooking spray. Add sandwiches to pan; cook 3 minutes on each side or until lightly browned and cheese melts.