Marthas Favorite Tuna Salad Sandwich

Name: Martha's Favorite Tuna Salad Sandwich
Ingredients: 12 ounces good-quality tuna, packed in oil, drained; 2 stalks celery, halved lengthwise and thinly sliced; 1 apple, such as McIntosh or Gala, peeled, cored, and cut into ¼-inch pieces; 3 tablespoons light mayonnaise, preferably Ojai Cook's Lemonaise Light; 2 tablespoons freshly chopped basil leaves; 1 tablespoon freshly squeezed lemon juice; Coarse salt and freshly ground pepper; 8 slices best-quality sourdough