Preheat the oven to 400F. Peel and cube the butternut squash, toss with olive oil, salt, pepper and nutmeg.
Place squash on a baking sheet and roast for 40 minutes, flipping half way through. Once squash is cooked through, mash with a potato masher or fork.
Cook the pasta according to the directions on the box.
While the pasta is cooking, cook the pancetta until crispy.
While the pancetta is cooking, heat a saucepan over medium heat. Add the butter and shallots, stirring until the butter browns. Add flour and which for 1-2 additional minutes.
Add the milk, mascarpone and squash; stir to combine.
Place the cooked pasta in a medium baking dish. Pour sauce over the pasta and add the cooked pancetta, spinach and rosemary. Fold the sauce into the pasta.
Top with shredded Mozzarella and Parmesan. Bake at 400F until cheese is golden and bubbling, about 25-30 minutes.