Use leftover chicken, onions, and honey mustard from our Grilled Honey-Mustard Chicken with Onions and Spinach Salad to make this baguette sandwich.
Thinly slice chicken breasts. Finely chop onions and combine with honey mustard and mayonnaise. Cut a baguette into 4 pieces and halve each lengthwise. Layer bottom halves with chicken, Swiss cheese, and onion mixture. Top with baby spinach and sandwich with top halves.