Tuna Macaroni Salad

Name: Tuna Macaroni Salad
Ingredients: Coarse salt and freshly ground pepper; 8 ounces elbow macaroni pasta; ½ small red onion, cut into ¼-inch pieces (about ½ cup); 2 ribs celery, cut into ¼-inch pieces (about 1 cup), plus ¼ cup celery heart leaves; ½ jalapeno, seeded if desired, and finely chopped (about 1 tablespoon); 1 (5-ounce) can solid tuna in water, drained and flaked; 6 tablespoons mayonnaise, preferably Hellmann's; ¼ cup low-fat buttermilk; 1 ½ teaspoons fresh lemon juice; 2 tablespoons dill relish; 1 teaspoon grainy mustard