Fill a large pot with about 2 quarts of water and plenty of salt (about 1 tablespoon). Bring to a boil, then cook your pasta according to the instructions on the box. Reserve 1 cup pasta water, then drain and set aside.
In a large skillet over medium-high heat, warm the olive oil. Add the mushrooms, spread them out into an even layer, then let them cook, undisturbed for 3-4 minutes, or until browned. Give the mushrooms a stir and cook for 2 more minutes, or until browned and cooked through. Sprinkle a pinch (⅛ teaspoon) salt all over and mix to combine. Transfer to a bowl to keep warm.
In the same skillet over medium-high heat, add your sausage, breaking it up into chunks and spreading out into an even later. Let the sausage cook for 2-3 minutes, undisturbed, or until browned. Give it a stir, breaking up any large chunks into smaller chunks, and let cook again, undisturbed for about 2 more minutes, or until cooked through.
In the same skillet over medium heat, add mushrooms, cooked pasta, cheese, ½ cup of the reserved pasta water, and a few cranks of black pepper. Stir everything together and let cook until the liquid reduces and a glossy sauce forms, stirring constantly, about 1-2 minutes. Once the sauce has reduced, add the butter and stir it in to coat until it’s melted into the sauce. If it starts to look dry, add in a splash of pasta water. Serve topped with more cheese, some black pepper, and chopped parsley (if using).