On one slice of bread, spread 1 tablespoon mustard, then place three slices cheddar, 4 to 5 apple slices, and three pieces cooked bacon. Top with another piece of bread. Repeat to make three more sandwiches.
Spread outsides of each sandwich with a thin layer of mayo.
Heat a large skillet over medium-low heat. Cook sandwiches until golden on both sides and cheese is melty, 3 to 4 minutes per side.
Serve immediately.
