1. Spray a nonstick pan with cooking spay and heat over medium-high heat.
2. Spread 1 tablespoon mustard on each of 4 slices of bread, then top each with 2 slices of cheese and 1 slice of tomato. Top with an additional slice of bread.
3. Place sandwiches in pan, and place another pan on top of the sandwiches.
4. Cook until bottoms of sandwiches are browned, 2-3 minutes. Flip and continue cooking an additional 2-3 minutes, until bread is golden and cheese is melted.