Drain the canned tuna thoroughly to remove excess moisture. Finely chop celery and red onion.
In a medium mixing bowl, combine tuna, mayonnaise, celery, red onion, and lemon juice. Stir gently until evenly mixed.
Add salt, black pepper, and Dijon mustard if using. Taste and adjust seasoning as needed.
Lay out 4 slices of bread. Evenly spread tuna salad over each slice. Add lettuce, tomato, and avocado if desired. Top with remaining bread slices.
Lightly butter the outside of each sandwich. Toast in a skillet over medium heat for 2–3 minutes per side until golden brown.
