Preheat oven to 350°. Heat oil in a large, ovenproof, nonstick frying pan. Sprinkle each chicken breast with 1/4 tsp. salt and lay in hot oil. Cook until browned on one side, about 5 minutes. Turn chicken over and transfer pan to oven.
Cook chicken until no longer pink in the center, about 10 minutes. Spread each breast with 1/2 tsp. mustard. Top each with ham, then cheese, dividing evenly. Return to oven until cheese melts. Serve with a green salad.
Note: Nutritional analysis is per serving.