Preheat oven to 375°F. Place bacon on a slotted broiler pan and bake to crisp.
Brush 1 side of each slice of bread liberally with melted butter.
Stir together Dijon mustard and maple syrup in a small bowl.
Assemble sandwiches: bread, buttered side out, maple mustard, cheddar, bacon, Gruyère, bread with mustard on inside, buttered side out.
Grill sandwiches on a nonstick griddle over medium heat until cheese melts and they’re deeply golden.