Grate the Parmesan very fine so it melts smoothly; set it aside.
Melt the butter in a medium saucepan over medium heat.
Stir in the garlic and cook until fragrant, about 30–45 seconds. Do not brown.
Pour in the heavy cream and bring to a gentle simmer. Cook, stirring, for 2–3 minutes.
Whisk in the cream cheese until the sauce is smooth and slightly thickened.
Lower the heat. Sprinkle in the Parmesan a handful at a time, whisking until fully melted before adding more.
Season with salt, pepper, and nutmeg. Remove from heat and let the sauce rest 1 minute to thicken.
Stir, adjust seasoning, and garnish with parsley. Toss with hot pasta or serve immediately.