Tomato, Tuna & Tarragon Salad

Ingredients

• 1/2 cup diced red onion
• 1/3 cup light mayonnaise
• 1/4 teaspoon kosher salt, or to taste
• 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 torn lettuce or mixed greens
• 1 pound small ripe tomatoes, cut into wedges
• 1 lemon, cut into 8 wedges