Boil fettuccine in well-salted water until al dente. Reserve 1/2 cup cooking water, then drain.
Melt butter in a large skillet over medium-low heat. Stir in garlic and cook until fragrant, about 30 seconds.
Pour in heavy cream and bring to a gentle simmer. Cook, stirring, until slightly thickened, 3 to 4 minutes.
Take the pan off heat. Gradually whisk in Parmesan and Romano until smooth and creamy.
Season the sauce with salt, black pepper, and a pinch of nutmeg.
Add the hot pasta to the pan and toss to coat, splashing in reserved water until glossy and silky.
Return to low heat for 1 minute, stirring until the sauce clings. Finish with parsley and serve right away.