In a small bowl stir together 1 tablespoon of the mayo and the mustard. Spread evenly over 4 slices of the bread. Arrange a slice of ham and a slice of Gruyere on each piece of bread.
Put the remaining 4 slices of bread on top of the sandwiches. Brush the outside of both slices of bread with the melted butter. Preheat an indoor grill or heat a nonstick grill pan over medium-high heat.
Grill for 2 to 3 minutes on each side or until the bread is lightly browned on both sides and there are nice-looking grill marks on the bread. Using a large spatula, transfer the sandwiches to a broiling tray or cookie sheet. Preheat the broiler.
Spread the top of the sandwiches with the remaining tablespoon mayo and top with the 4 remaining slices cheese, being sure to cover the crusts of the bread.
Broil, cheese side up, for 1 to 2 minutes, or until bubbly and golden brown. Serve hot.