Bring a large pot of salted water to a boil. Add the fettuccine and cook until al dente, about 8–10 minutes. Reserve 1 cup of pasta cooking water before draining.

In a large skillet over medium-low heat, melt the butter. Stir in the heavy cream, whisking until smooth. Simmer gently for 2–3 minutes.

Gradually whisk in the grated Parmesan and Romano cheeses until the sauce thickens. Season with salt and pepper.

Add the drained fettuccine to the skillet, tossing to coat. Use reserved pasta water to thin the sauce, if needed.

Garnish with chopped parsley. Serve immediately for the creamiest, most decadent Alfredo.