Cook pasta shells according to package directions. Meanwhile in a saucepan, combine the flour, milk, dill and salt until smooth. Bring to a boil; cook and stir until thickened, about 2 minutes. Pour 1-1/4 cups sauce into a 2-qt. baking dish; set aside. , In a nonstick skillet, saute celery and onion in oil until tender. Add bread. Stir in the tuna, salad dressing and cheese; mix well. Drain shells; stuff with tuna mixture. Place over sauce. Drizzle with remaining sauce. Cover and bake at 350° until bubbly and heated through, 25-30 minutes.