Coat a medium-size nonstick skillet with nonstick cooking spray and place over medium heat. Add onion, sugar and salt to skillet; cover. Cook for 10 minutes, stirring occasionally, or until lightly browned; remove from heat.
Stir together mustard, horseradish and honey; set aside.
Spread 4 slices of bread evenly with mustard mixture, then layer 3 slices ham over each piece of bread. Place about 2 tablespoons onions on top of the ham, then place 1 slice cheese on top of the onions. Place 4 remaining bread slices over cheese.
Spritz both sides of sandwich with nonstick cooking spray and cook in a nonstick skillet over medium heat for 5 to 8 minutes or until cheese has melted, turning halfway through. Serve with mixed greens on the side, if desired.