Makes: 4 servings. Prep: 5 minutes. grill: 6 minutes. Broil: 2 minutes.
1. In a small bowl, stir together 1 tablespoon of the mayonnaise and the mustard. Spread evenly over 4 slices of bread. Arrange a slice of ham and a slice of Gruyere on each piece of bread.
2. Put the remaining 4 slices of bread on top of the sandwiches. Brush the outside of both slices of bread with the melted butter.
3. Heat an indoor grill or heat a nonstick grill pan over medium-high heat.
4. 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.
5. Heat the broiler.
6. Spread the top of the sandwiches with the remaining tablespoon mayonnaise and top with the 4 remaining slices Gruyere, being sure to cover the bread crusts.
7. Broil, cheese side up, for 1 to 2 minutes until bubbly and golden brown. Serve hot.