Tuna and Apple Salad Sandwiches

1/3 cup mayonnaise
    1/4 cup finely chopped celery
    1/4 cup finely chopped walnuts
    2 tablespoons finely chopped onion
    1 tablespoon sweet pickle relish
    1 teaspoon sugar
    1 pouch (7.1 ounces) packed tuna
    1/2 cup chopped red apple
    6 slices bread, toasted
    6 lettuce leaves

 In a large bowl, combine the first six ingredients; stir in tuna and apple. Spread 1/2 cup tuna mixture on three slices of bread. Top with lettuce and remaining bread. Yield: 3 servings.