In small bowl, combine tuna, seasoning, celery, mayonnaise & shredded cheddar.
Spread 2 slices of bread with a very thin layer of mayonnaise. Place spread side down in skillet.
Divide tuna mixture between the two slices of bread in the skillet; top with slice of cheese.
Spread remaining bread slices with very thin layer of mayonnaise. Place spread side up on top of cheese.
Cook over medium-high heat (watch closely) until browned on both sides, turning once during cooking.