Combine chicken, 1/4 cup Hellmann's® or Best Foods® Real Mayonnaise and onion in large bowl.
Spread chicken mixture on 4 bread slices, then top with cheese. Evenly spread remaining 4 bread slices with mustard, then place on sandwiches, mustard-side-down. Coat outside of sandwiches with remaining Mayonnaise.
Cook sandwiches in large nonstick skillet or grill pan over medium heat, turning once, 6 minutes or until sandwiches are golden brown and cheese is melted.