Fill a large pot with 4-6 quarts of water, bring to a rolling boil over high heat. Add 1-2 tablespoons salt. Add the fettuccine and cook until al dente according to package instructions (typically 8-10 minutes). Reserve 1 cup of pasta cooking water, then drain the pasta and set aside.

Prepare and cook the chicken

While water heats, pat chicken breasts dry and season both sides with 1 tsp kosher salt and 1 tsp black pepper. If breasts are thick, butterfly or pound to even thickness (~3/4 inch). Heat a large skillet over medium-high heat and add 1 tbsp olive oil and 1 tbsp unsalted butter. When the pan is hot and butter melted, add chicken and cook 4-6 minutes per side until golden brown and internal temperature reaches 165°F (74°C). Transfer chicken to a cutting board and let rest 5 minutes, then slice into 1/2-inch strips or bite-sized pieces.

Alfredo Sauce

Melt butter and soften garlic

In the same large skillet (wipe out any large browned bits leaving some fond for flavor), reduce heat to medium-low and add 6 tbsp unsalted butter. Once butter is melted, add the minced garlic and cook 30-45 seconds until fragrant but not browned.

Add cream and milk

Pour in 1 cup heavy cream and 1/2 cup whole milk, stirring to combine. Increase heat slightly to bring the mixture to a gentle simmer (do not boil). Simmer gently for 3-4 minutes, stirring occasionally, so the cream reduces slightly and thickens.

Incorporate Parmesan

Lower heat to low. Gradually whisk in 1 1/2 cups freshly grated Parmesan, a handful at a time, until the cheese is fully melted and the sauce is smooth. If the sauce seems too thick, whisk in up to 1/2 to 1 cup reserved pasta cooking water a little at a time to reach a silky consistency.

Season sauce

Season the Alfredo with 1/2 tsp salt, 1/2 tsp freshly ground black pepper, and a pinch of freshly grated nutmeg if using. Taste and adjust seasoning as needed.

Finish & Serve

Combine pasta, chicken, and sauce

Add the drained fettuccine directly into the skillet with the Alfredo sauce (or transfer sauce to the pot with pasta). Toss gently to coat the noodles evenly. Add sliced chicken and continue tossing or gently folding until heated through. If sauce needs loosening, add reserved pasta water 1-2 tablespoons at a time until desired consistency is reached.

Garnish and serve

Turn off heat and stir in 2 tbsp chopped fresh parsley. Divide among 4 plates or shallow bowls, sprinkle with extra grated Parmesan to taste (about 1/4 cup total), and finish with a crack of black pepper. Serve immediately.