- ¼ white onion, thinly sliced
- Small red onion, thinly sliced
- 2 Tbsp extra-virgin olive oil
- 1 Tbsp red-wine vinegar, plus more to taste
- ¼ tsp salt
- Black pepper to taste
- 1 (6 oz) can tuna in olive oil
- Lemon juice, to taste
- 2 Kaiser rolls
- Lettuce leaves
- 8 tomato slices
- 1 egg, hard-boiled
1. Combine onions with olive oil, 1 Tbsp red-wine vinegar, salt, and black pepper in a bowl; marinate 5 minutes.
2. Mix in tuna, including the oil. Season with lemon juice, more red-wine vinegar, salt, and pepper to taste.
3. Cut kaiser rolls in half crosswise; place lettuce leaves on bottom half. Top each with 4 tomato slices, half the tuna mixture, and 3 slices hard-boiled egg. Add pitted Niçoise olives, if desired.