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 T 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 nonstick skillet over medium heat for 5-8 minutes or until cheese has melted, turning halfway through.