Preheat dual contact grill. Combine honey, Djon mustard, and thyme leaves. Brush on onion slices. Grill onion slices for 3-5 minutes until tender and marked with grill marks. Remove from grill and set aside.
Grill ham slices for 3-4 minutes until heated through. Spread butter on one side of each piece of bread. Make sandwiches with the grilled onions, ham, and cheese, assembling sandwiches with buttered side out. Grill each sandwich for 3-6 minutes until hot and cheese is melted