01 - Cook pasta in a large pot of salted boiling water until al dente, following package instructions. Reserve 1/2 cup pasta water, then drain.
02 - Melt butter in a large skillet over medium heat. Add chopped onion and sauté for 3 minutes until translucent.
03 - Add minced garlic to the skillet and cook for 1 minute until aromatic.
04 - Stir in spinach and cook for 2 to 3 minutes, just until wilted.
05 - Pour in heavy cream and milk, bringing mixture to a gentle simmer.
06 - Add grated Parmesan and ground nutmeg. Stir constantly until cheese melts and sauce thickens, about 2 to 3 minutes. Season with salt and pepper.
07 - Add cooked pasta to the skillet and toss to coat. Adjust sauce consistency with reserved pasta water, as needed.
08 - Transfer to plates and garnish with extra Parmesan and freshly ground black pepper if desired.