Place chicken between two pieces of plastic wrap. Pound until uniform thickness.

Preheat grill following manufacturer's instructions.

Place chicken on grill and cook until no longer pink and juices run clear.

Season with salt, pepper, paprika, and garlic powder.

Carefully, spoon 1 Tablespoon mustard on each chicken breast and smooth to edges.

Top each chicken breast with 2 slices of ham, torn or folded to fit. Place one slice of cheese on top of ham. Close grill lid and allow cheese to melt for 2-3 minutes.

Serve on a bun with onion slices.