Spread about 1 teaspoon mustard over cut sides of rolls. Arrange 1 slice cheese, 1 1/2 ounces ham, 1 1/2 teaspoons relish, and 1 1/2 ounces turkey over 4 roll halves. Top each serving with 1 slice cheese. Cover with top halves of rolls.
Heat a nonstick skillet over medium heat. Coat pan with cooking spray. Add sandwiches. Place a heavy cast-iron skillet on top of sandwiches; cook 6 minutes on each side or until golden brown. Cut each sandwich into 6 wedges.