Italian Tuna Salad

Name: Italian Tuna Salad
Ingredients: 1/4 cup thinly sliced red onion (from 1 small onion); 1/4 cup red wine vinegar; 1 1/2 tablespoons fresh lemon juice (from 1 lemon); 1 tablespoon Dijon mustard; 1 teaspoon honey; 3/4 teaspoon kosher salt; 1/2 teaspoon black pepper; 5 tablespoons extra-virgin olive oil, divided; 3 ( 6.7-ounce) jars oil-packed tuna (such as Tonnino), drained and flaked into large pieces; 1 ( 6.7-ounce) jar pitted Castelvetrano olives , drained and roughly chopped (about 1 cup; 1 (8 1/2-ounce) jar oil-packed sun-dried tomato halves , drained and roughly chopped (about 1 packed cup); 3 large celery stalks with leaves, thinly sliced (about 1 1/4 cups); 1/2 cup loosely packed fresh basil leaves , roughly chopped (from 1 [1/2-ounce] package)