In a large mixing bowl, combine the drained tuna, diced celery, chopped red onion, and sweet pickle relish.

Add the mayonnaise, lemon juice, Dijon mustard, salt, and black pepper. Mix well until all ingredients are combined.

Taste and adjust seasoning if necessary.

If using, gently fold in cherry tomatoes and diced avocado.

Serve on a bed of mixed greens or in a sandwich.