01 - Bring a large pot of salted water to a boil. Cook the fettuccine according to package directions until al dente. Drain, reserving ½ cup of the pasta water.
02 - Pat chicken breasts dry and season both sides with salt and black pepper.
03 - Heat olive oil in a large skillet over medium-high heat. Cook chicken breasts 5 to 6 minutes per side until golden brown and cooked through. Transfer to a plate, rest for 5 minutes, then slice thinly.
04 - Reduce skillet heat to medium. Add butter and minced garlic; sauté for 1 minute until aromatic.
05 - Pour in heavy cream and bring to a gentle simmer. Stir in Parmesan and Pecorino cheeses, ground nutmeg, salt, and pepper. Simmer for 3 to 4 minutes, stirring constantly, until sauce thickens.
06 - Add cooked fettuccine to the sauce and toss to coat evenly. If necessary, add reserved pasta water gradually until desired consistency is achieved.
07 - Top pasta with sliced chicken. Garnish with chopped parsley and additional grated Parmesan cheese.