Rinse lettuce leaves and refrigerate on small plate.
Slice tomatoes and cheese. Set aside on small plate
Spread butter or spray one side of each bread slice. Set aside.
Chop celery and onion, combine with tuna, mayonnaise, lemon, and relish in a small bowl. Fold together until well mixed.
Pre-heat large skillet over medium heat.
On non-buttered side of bread slices, evenly spread tuna mixture on 4 slices.
Top tuna mixture with a slice of cheese, 2 tomato slices and slice of bread, buttered side up.
With spatula, transfer sandwiches, tomato side up, to pre-heated skillet at medium heat. Cook for 3-4 minutes or until cheese is melted and bottom or bread is golden brown. Flip sandwiches over and repeat till other side is golden brown.
As sandwiches are done, transfer to serving plates, lift bread slice, top with a lettuce leaf; replace bread slice. With bread knife, slice in half, diagonally, and serve.
Steps 1-4 can be prepared up to a couple hours in advance, sealed and refrigerated till ready to make the sandwiches.
Tips, Notes, and Variations:
I like to prepare the lettuce a little in advance and refrigerate damp to get back some of that store fresh “crispness”.
If you’d rather not grill with butter/oil, these sandwiches can also be baked. Pre-heat oven to 350°F. Place sandwiches on cookie sheet and bake for 12-15 minutes till cheese is melted and bread has lightly browned. Baking is also a preferred method if preparing a large number of sandwiches or wishing to use large Croissants, split in half, for the bread.
Tuna Salad Variations:
If you'd rather not have a grilled tuna sandwich, here're variations to the basic recipe for your favorite Tuna Salad Sandwich.
Use yogurt instead of mayo for a lighter salad.
Use extra-virgin olive oil instead of mayo for a looser salad.
Use canned salmon, smoked trout, or leftover chicken in place of the tuna.
Add diced apples, raisins, or dried fruit to the basic recipe.
Add diced fennel, shredded cabbage, diced cucumbers, or any other crunchy vegetable.
Add a dash of curry powder, paprika, dukkah, harissa, chopped fresh herbs, or any other spice mix in your cupboard.
Tuna and mayo — it's as simple as you can get, and a perfectly acceptable lunch.