01 - Bring a large pot of salted water to a boil. Cook pasta until al dente according to package directions. Reserve 1/2 cup pasta water before draining.
02 - Heat olive oil and butter in a large skillet over medium heat. Add diced onion and cook 2–3 minutes until translucent.
03 - Stir in minced garlic and cook for 1 minute until fragrant, stirring constantly to prevent burning.
04 - Add spinach with a pinch of salt. Cook 2–3 minutes, stirring occasionally, until completely wilted.
05 - Reduce heat to low. Pour in heavy cream and stir in Parmesan cheese. Add nutmeg if desired. Simmer gently 2–3 minutes until sauce thickens slightly.
06 - Add drained pasta to the skillet, tossing well to coat. Add reserved pasta water as needed for desired sauce consistency.
07 - Season with salt and freshly ground black pepper to taste. Serve immediately with extra Parmesan on top if desired.