Stir together 1 tablespoon of the mayonnaise and the mustard. Spread it over 4 slices of bread. Add a slice of ham and a slice of Gruyère to each, then close the sandwiches with the remaining bread. Brush the outsides with melted butter.

Heat an indoor grill or nonstick grill pan over medium-high heat. Cook the sandwiches for 2–3 minutes per side, until lightly browned. Transfer to a baking sheet and preheat the broiler.

Spread the remaining tablespoon of mayonnaise over the tops of the sandwiches. Add the remaining 4 slices of Gruyère, covering the bread right to the edges. Broil, cheese side up, for 1–2 minutes, until bubbly and golden. Serve hot!