Spread mustard on 4 slices of bread. Top with ham, sliced cheese, and remaining 4 bread slices.

Whisk milk and egg whites together in a shallow dish or pie plate. Coat a nonstick griddle (or skillet) with cooking spray and heat over medium. Dip each side of the sandwiches in egg mixture. Cook for 1-2 minutes per side or until golden brown and cheese is melted.

Serve sandwiches with cubed melon and cucumber slices.