Rinse the mushrooms under tap water and dry with paper towels. Remove stems and cut the portobello into 3 parts. Cut all the mushrooms into ¼-inch slices. Transfer to a large bowl and set aside.
Bring a large pot of water (salted) to a boil over high heat. Cook the pasta until tender but still firm to the bite, stirring occasionally. Reserve 1 cup of pasta water. Drain pasta. Set aside.
Meanwhile, in a large skillet over medium-high heat, add pancetta. Cook until golden brown, stirring occasionally. Transfer to a plate lined with paper towels.
In the same skillet, remove most of the fat, except for 1 tablespoon. Add onion and cook for 3 minutes. Add garlic and cook for 1 more minute. Add mushrooms and cook over medium-high heat until they are slightly golden brown, stir occasionally, about 10 minutes. Stir in 2 tablespoons butter until melted. Add parmesan, pasta, and ¾ cup pasta water. Stir to combine. Season with salt and pepper. Transfer to serving plates. Sprinkle with extra parmesan cheese and parsley. Serve immediately.