01 - Bring a large pot of salted water to a boil. Cook the pasta according to package instructions until al dente. Reserve ¼ cup of pasta water, then drain and set aside.
02 - While the pasta cooks, heat butter and olive oil 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, stirring constantly to prevent burning.
04 - Add the spinach and cook until wilted, about 2-3 minutes.
05 - Pour in the heavy cream, bring to a gentle simmer, and stir in the grated Parmesan cheese. Season with salt, pepper, and nutmeg (if using). Simmer for 2-3 minutes, stirring occasionally.
06 - Toss the cooked pasta into the sauce, adding reserved pasta water as needed to reach desired creaminess. Serve immediately, garnished with extra Parmesan, black pepper, and chopped parsley if desired.