01 - Bring a large pot of salted water to a boil. Cook pasta according to package directions until al dente. Reserve 1/2 cup pasta cooking water, then drain.
02 - Heat olive oil in a large skillet over medium heat. Add diced onion and sauté for 2 to 3 minutes until softened. Stir in minced garlic and cook for 1 minute until fragrant.
03 - Add chopped spinach to the skillet and sauté for 2 to 3 minutes until wilted.
04 - Pour in heavy cream, reduce heat to low, and stir in grated nutmeg. Simmer gently for 2 minutes.
05 - Stir in grated Parmesan until sauce is smooth. Season with salt and black pepper to taste.
06 - Add drained pasta to the skillet and toss to coat evenly. Incorporate reserved pasta water as needed to loosen the sauce.
07 - Serve immediately, garnished with extra Parmesan, lemon zest, crushed red pepper flakes, and fresh basil if desired.