Preheat oven to 350°. Cook pasta al dente; drain. Meanwhile, crumble sausage into a large frying pan and cook over medium heat, breaking up with a wooden spoon, until well browned, about 10 minutes. Pour into a large bowl.
Increase heat to medium-high. Add oil and fennel seeds and let sizzle a few seconds. Stir in onion, chile, celery, sage, thyme, rosemary, nutmeg, and salt and pepper to taste. Sauté until onion is just beginning to soften, then stir in garlic and cook until onion is softened, about 10 minutes total.
Add vegetables and cooked pasta to sausage, then season to taste with salt and pepper. Stir in eggs and parsley leaves.
Pour pasta into a shallow 3- to 4-qt. baking dish. Pour in broth (it should come about two-thirds of the way up the pasta). Sprinkle with Parmesan.
Cover dish with foil and bake on middle or lower rack until broth is mostly absorbed and everything is piping hot, 30 to 45 minutes. Uncover, increase heat to 450°, and cook until cheese begins to bubble and brown, 10 minutes more.
Sprinkle with chopped parsley if you like, and serve hot.
*Store-bought bone broth (or homemade stock) is rich and thick, making a tastier stuffing than ordinary store-bought chicken broth.
MAKE AHEAD: Two days, covered and chilled. Reheat at 350°, covered with foil, until heated through, about 20 minutes.