Bring a large pot of generously salted water to a boil for the pasta.
Melt the butter in a heavy-bottomed saucepan over medium-low heat. Add the minced garlic and cook, stirring, for 60–90 seconds until fragrant and just golden.
Add the cubed cream cheese and whisk continuously until fully melted and smooth, about 2 minutes.
Pour in the heavy cream, whisk to combine, and bring to a gentle simmer. Reduce for 8 minutes, stirring every minute.
Remove the pan completely from the heat. Add the parmesan and romano in three additions, whisking until fully melted between each addition.
Whisk in the nutmeg, salt, and black pepper to taste.
Cook the fettuccine according to package directions until al dente. Reserve 1 cup of pasta water, then drain.
Add the hot, drained pasta directly to the sauce and toss with tongs to coat. Loosen with reserved pasta water as needed.
Garnish with fresh parsley and serve immediately.