Combine stone-ground and Dijon mustards. Spread mustard mixture evenly onto each of 4 bread slices. Top each evenly with ham, cheese, tomato, and red onion. Cover with remaining 4 bread slices.
Place a large nonstick skillet coated with cooking spray over medium-high heat. Add 2 sandwiches to pan; cook 4 minutes on each side or until lightly browned. Repeat procedure with remaining 2 sandwiches. Serve immediately.