This is the sandwich I make almost every time I have leftover chicken in the fridge. It's fresh, flavorful and only takes a few minutes to put together.
Mix the mustard and mayo together in a small bowl.
Spread the mixture onto each slice of bread, add a couple pieces of sliced chicken, then top with spinach and arugula, tomato and onion. Sprinkle lightly with coarse salt and black pepper and serve.