Pre-heat the oven to 375°F. Place the bacon on a slotted broiler pan and bake to crisp.
Brush one side of each slice of bread liberally with melted butter.
Stir together the Dijon mustard and maple syrup in a small bowl.
Assemble the sandwiches: bread, buttered side out, maple mustard, cheddar, bacon, Gruyère, bread with mustard on inside, buttered side out.
Grill the sandwiches on a nonstick griddle over medium heat until the cheese melts and they’re deeply golden.