Place the chicken breasts between two sheets of plastic wrap and pound to an even thickness of about 1/2-inch. Rub the top side of the chicken pieces with half of the mustard, then sprinkle with half of the cheese (pressing to adhere), and season with half of the pepper. Flip the chicken pieces over and repeat.
Spray a nonstick skillet with cooking spray, and heat over medium-high heat. Add the chicken to the pan, and cook, without moving it, until the cheese on the bottom forms a deep brown crust that releases fairly easy from the bottom of the pan (3 to 4 minutes). Flip and repeat on the other side, cooking until the chicken is cooked through, about three more minutes.
Notes
*If you are preparing this recipe as GLUTEN-FREE, just make sure that the brand of Dijon you are using is known to be GF.