In a medium bowl mix together the tuna, mayonnaise, celery, onions, lemon juice, salt, and pepper.
Spread the butter on one side of each slice of bread.
Place four slices of bread in a pan over medium heat butter side down. Place two slices of cheese on each slice of bread. Top the cheese with the tuna mixture. Place the bread on the top of the tuna butterside out.
Cook for 2-3 per side or until the bread is golden brown. Then flip the sandwiches and cook for an additional 2-3 minutes.