Bring a large pot of salted water to a boil. Add pasta and cook until just shy of al dente. Reserve at least 1 cup of the pasta cooking water before draining.

While the pasta cooks, heat olive oil in a large skillet or Dutch oven over medium heat. Add diced onions and sauté for 2–3 minutes until just starting to soften. Add garlic and cook 30 seconds more, until fragrant.

Stir in the finely chopped mushrooms. Cook for 5–6 minutes, until most of their moisture has evaporated and they begin to brown.

Add the sausage directly to the onion and mushroom mixture. Use a wooden spoon or spatula to mash and break up the sausage as it cooks. Let it brown and cook through, about 8–10 minutes.

Pour in the white wine and scrape up any brown bits from the bottom of the pan. Let it simmer for 2–3 minutes until slightly reduced.

Reduce heat to medium-low. Stir in the heavy cream (or ricotta). Add nutmeg, black pepper, and rosemary if using. Let the sauce gently simmer for 2–3 minutes, allowing everything to meld.

Add the drained pasta to the skillet. Stir gently to coat the pasta in the creamy sausage mixture, adding reserved pasta water as needed to loosen and emulsify the sauce.

Turn off heat and stir in grated Pecorino Romano. Taste and adjust seasoning with salt and black pepper.