Tuna Salad with Tomato and Tarragon

Ingredients

• 1/2 cup diced red onion
• 1/3 cup reduced-fat mayonnaise
• 1/4 teaspoon sea salt
• Freshly ground pepper to taste
• 2 6-ounce cans chunk light tuna in olive oil, drained
• 2 stalks celery, thinly sliced (about 1 cup)
• 1/4 cup packed coarsely chopped fresh tarragon leaves
• 8 cups mixed greens
• 1 pound small ripe tomatoes, cut into wedges