Tuna Salad with Apples, Walnuts, and Tarragon

Ingredients

• 1/4 cup finely chopped onion
• 2 tablespoons olive oil
• 3 (5-ounce) cans solid white tuna in water
• 1 apple, cored and cut into 1/2-inch pieces
• 1/2 cup walnuts, toasted and chopped coarse
• 1/2 cup plus 2 tablespoons mayonnaise
• 1 celery rib, minced
• 1 tablespoon minced fresh tarragon
• 2 teaspoons lemon juice
• 1/2 teaspoon sugar
• 1/2 teaspoon table salt
• 1/2 teaspoon black pepper