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