Preheat broiler. Cook turkey bacon in medium nonstick skillet until crisp, about 5 minutes. Drain and set aside.
Brush all 8 slices of toast with 1 tsp of mustard each. Place 4 slices of toast, mustard-side up, on broiler pan. Top toast with apple slices and onion slices.
Sprinkle cheddar over onion and broil 4 inches (10 cm) from heat until cheese has melted, about 2 minutes.
Top each sandwich with two pieces of bacon and remaining toast. Cut in half.