Spread 1 teaspoon mustard on each of 4 bread slices; top each of 2 bread slices with 1 Cheddar cheese slice, 2 tomato slices, and 1 fontina cheese slice. Top with remaining bread slices.
Melt butter in a large nonstick skillet over medium heat. Lightly coat both sides of sandwiches with cooking spray. Place sandwiches in pan; cook 3 to 4 minutes on each side or until golden brown. Serve immediately.