Cook spaghetti according to package instructions. Reserve 1/4 cup pasta water, then drain and set aside.
In a large skillet, cook chopped bacon over medium heat until crispy. Remove and set aside.
In the same skillet, add onions and sauté until translucent. Add garlic and cook for another minute.
Lower the heat and stir in sour cream, heavy cream, mustard, and nutmeg (if using). Mix well.
Add the cooked pasta to the skillet and toss to coat, adding reserved pasta water if needed to loosen the sauce.
Stir in the crispy bacon, season with salt and pepper, and cook for 1–2 minutes more.
Serve warm, garnished with chopped parsley and grated cheese if desired.