Stir flaked tuna, ¼ cup yogurt, 3 tablespoons celery, 2 tablespoons each mayonnaise, pickles and onion, 1 tablespoon parsley, 2 teaspoons each mustard and lemon juice, ¼ teaspoon crushed red pepper and ⅛ teaspoon salt together in a small bowl until well combined.
Divide the tuna mixture between 2 toast slices. Top with avocado slices, lettuce, tomato slices and the remaining 2 toast slices. Cut into halves or quarters.
