01 - Bring a large pot of salted water to a boil. Cook the pasta according to package instructions until al dente. Reserve 1/2 cup of pasta water, then drain and set aside.
02 - While the pasta cooks, melt butter in a large skillet over medium heat. Add the onion and sauté for 2–3 minutes until translucent.
03 - Add the minced garlic and cook for 1 minute until fragrant.
04 - Stir in the spinach and cook for 2–3 minutes until wilted.
05 - Pour in the heavy cream, bring to a gentle simmer, and add the grated Parmesan cheese. Stir until the cheese melts and the sauce thickens slightly, about 2 minutes. Add nutmeg, salt, and pepper to taste.
06 - Add the cooked pasta to the sauce, tossing to coat evenly. If the sauce is too thick, add a splash of reserved pasta water to loosen.
07 - Serve immediately, topped with extra Parmesan and freshly cracked black pepper.