Bring 4 liters (about 1 gallon) of water to a rolling boil in a large pot. Add 2 tablespoons kosher salt. Add 12 oz fettuccine and cook according to package instructions until al dente (usually 8–12 minutes for dried, 2–4 minutes for fresh). Reserve 1 to 1 1/2 cups of pasta cooking water, then drain pasta and set aside. Do not rinse.

Alfredo sauce

Warm the dairy

In a large sauté pan or shallow skillet over medium-low heat, melt 6 tablespoons unsalted butter. Add the minced garlic and cook gently 30–45 seconds until fragrant, taking care not to brown it.

Add cream and milk

Pour in 1 cup heavy cream and 1 cup whole milk and warm until small bubbles form at the edge (do not boil). Reduce heat to low.

Incorporate cheeses

Add 1.5 cups freshly grated Parmesan and 2 ounces cream cheese (if using) in small handfuls, stirring constantly and allowing each addition to melt before adding the next. Continue stirring until the sauce is smooth and slightly thickened, 3–5 minutes.

Season

Season the sauce with 1 teaspoon freshly ground black pepper, 1/2 teaspoon salt (adjust to taste), and a pinch of freshly grated nutmeg if desired. Keep the sauce at a very low simmer — it should be warm and glossy, not boiling.

Finish & combine

Combine pasta and sauce

Add the drained al dente fettuccine to the skillet with the sauce. Toss gently with tongs or two forks to coat. If the sauce seems too thick or clings without coating evenly, add reserved pasta water a tablespoon at a time until you reach a silky, clingy consistency (usually 1/4 to 3/4 cup).

Adjust and finish

If using, stir in 1 tablespoon unsalted butter for extra sheen. Taste and adjust seasoning with additional salt and pepper. If desired, stir in up to 2 tablespoons more grated Parmesan to deepen the flavor.

Plate and garnish

Divide the fettuccine Alfredo among warmed plates or bowls. Sprinkle with 1/4 cup additional grated Parmesan and 1 tablespoon chopped fresh parsley. Serve immediately.