Preheat oven to 400F.
Boil a pasta of salted water. Cook the pasta according to package directions, until al dente.
In the meantime, add 1 tablespoon olive oil in a pan on medium high heat. Add in the finely diced sweet onion and saute for 5 minutes, stirring occasionally. Reduce heat to medium, then add in the chopped sage leaves and cook for an additional 5-10 minutes, until slightly caramelized.
In a large bowl, mix together the pumpkin puree, sea salt, black pepper, nutmeg, chicken broth or vegetable stock. Toss in the caramelized onions and cooked pasta.
Transfer the pasta mixture into a casserole dish and bake covered for 10 minutes.
Take the pasta bake out of the oven and top with the grated cheese and slices of mozzarella. Bake uncovered for 10-15 minutes, until cheese is melted. (Optional: Broil for 1-3 minutes for extra crispiness)