Salade Niçoise Sandwich

INGREDIENTS

• 1/2 small red onion, peeled and thinly sliced
• 2-3 radishes, thinly sliced
• 12 Niçoise olives
• 1 1/2 tablespoons capers
• 6 anchovy filets (optional)
• 1 tomato, cut into wedges
• 3 small red potatoes, scrubbed
• 3 hard boiled eggs, peeled and quartered
• 2 tablespoons red wine vinegar
• 1 6-ounce can tuna, drained
• Salt and freshly ground black pepper
• 4-5 cups cleaned and dried mixed lettuces
• 7 tablespoons extra virgin olive oil
• 1 teaspoon Dijon mustard