1 5 ounce can solid white tuna in water, drained and flaked with a fork
1 stalk celery, peeled, trimmed, sliced in half lengthwise, and thinly sliced, about ⅓ cup
¼ medium apple, cut into ¼-inch dice, about ⅓ cup
1 green onion, trimmed and thinly sliced
1 to 2 teaspoons chopped fresh tarragon, dill, or parsley, optional
Zest ¼ lemon, plus more to taste
3 to 4 tablespoons mayonnaise
1 teaspoon Dijon mustard
Kosher salt and pepper
4 slices whole wheat bread, lightly toasted
½ avocado, seeded and peeled
2 Bibb lettuce leaves