Tuna Nicoise Sandwiches

Name: Tuna Nicoise Sandwiches
Ingredients: 1/2 white onion, thinly sliced; 1/2 red onion, thinly sliced; 1/4 cup plus 4 teaspoons extra-virgin olive oil; 2 tablespoons red wine vinegar; Coarse salt and freshly ground pepper; 2 jars (7 ounces each) best-quality tuna packed in olive oil; 1 tablespoon fresh lemon juice, plus lemon wedges, for serving; 4 small ciabatta rolls, split horizontally; 2 heads Bibb lettuce, leaves separated; 1/2 cup nicoise olives, pitted; 4 hard-boiled eggs, peeled and thinly sliced