- Place the tuna, celery, apple, onion, tarragon as desired, and lemon zest in a bowl and stir to combine. Mix in the mayonnaise, mustard, and salt and pepper to taste.
- Lay 2 of the bread slices on a work surface. Use the back of a fork to smash and spread the avocado evenly over each. Top evenly with the lettuce, tuna mixture, and remaining slice of bread. Slice each in half and serve.
Other ways of serving include as an open-face sandwich, over a tossed green salad, or scooped onto a halved avocado.