Bring a large pot of salted water to a rolling boil. Cook mafalda according to package directions until al dente. Just before draining, reserve 1 cup of starchy pasta water, then drain the pasta and set aside.

In a large deep skillet or Dutch oven, heat olive oil over medium-high. Add the sausage, breaking it into small pieces with a wooden spoon. Cook until deeply browned and crumbled, and the internal temperature reaches 160°F (71°C), about 5-7 minutes. Transfer to a plate with a slotted spoon, leaving the rendered fat in the pan.

In the same skillet, add the sliced mushrooms and a pinch of salt. Cook undisturbed for 3 minutes to let them sear, then stir and continue cooking until golden, about 5 minutes total. Add the chopped onion and cook until translucent, 3-4 minutes. Stir in the minced garlic until fragrant, 30 seconds.

Pour in the white wine (or broth) and scrape up any browned bits from the bottom of the pan. Let it bubble and reduce by half, about 2 minutes. Return the browned sausage to the pan, then pour in the heavy cream. Bring to a gentle simmer and cook for 5 minutes, letting the sauce thicken slightly.

Add the drained mafalda pasta and 1/2 cup of the reserved pasta water to the sauce. Toss vigorously with tongs until the pasta is well coated and the sauce is glossy. Stir in the grated Parmesan and a pinch of nutmeg (if using). If the sauce seems too thick, add more pasta water a splash at a time.

Taste and adjust seasoning with salt and plenty of black pepper. Transfer to a warm serving platter or bowls. Shower with extra Parmesan and a flurry of chopped parsley. Serve immediately.