Tuna Avocado Club

Name: Tuna Avocado Club
Ingredients: 2 (6-ounce) cans water-packed tuna, drained; 1 Kirby cucumber, seeded and finely diced (3/4 cup); 3 tablespoons extra-virgin olive oil, plus a drizzle; 3 tablespoons whole-grain mustard; 2 scallions (white and green parts), thinly sliced; 1 1/2 tablespoons chopped fresh dill leaves; 1 1/2 teaspoons finely grated lemon zest from 1 lemon; Kosher salt; Freshly ground black pepper; 8 slices nine-grain bread; 1 Hass avocado, halved, seeded, and sliced; 1 1/2 cup mesclun salad, or other lettuce; 2 large tomatoes