Preheat oven to 350°. In a small saucepan, saute onion and celery in butter until tender. Combine flour and broth until smooth; stir into the pan. Add milk, bay leaf, onion salt and pepper. Bring to a boil; cook and stir 2 minutes or until thickened. Remove from heat. Discard bay leaf. Whisk in sour cream, parsley and lemon juice.
Place the spaghetti in a greased 11x7-in. baking dish; top with pork and white sauce. Sprinkle with bread crumbs. Bake, uncovered, 30-35 minutes or until bubbly. Let stand 5 minutes before serving.