Bring 12 cups water to a boil in a large pot. Add 1 tablespoon fine sea salt. Add fettuccine and cook to al dente, going with the shortest time listed on the package. Before draining, scoop out 1 cup pasta water. Drain pasta; do not rinse.
Slice chicken breasts in half widthwise to make 4 thinner filets. Lightly pound so pieces are even in thickness. Season both sides with 1/4 tsp salt, 1/4 tsp pepper, 1/2 tsp garlic powder, and Italian seasoning.
In a very large nonstick skillet (one that can hold all the pasta later), add 1 tbsp butter and olive oil. Heat over medium-high. Once the butter is melted, add the four chicken pieces in an even layer. Cook for 3 minutes, flip, then cook another 2 to 4 minutes or until cooked through. Remove to a plate and rest 5 minutes before thinly slicing or dicing.
Cut remaining 6 tbsp butter into 6 pieces. In the same pan, melt butter over medium heat. Stir in onion powder and garlic powder, scraping up any browned bits on bottom of pan. Cook about 1 minute. Pour in cream and add remaining 1/4 tsp salt & 1/4 tsp pepper. Bring to a gentle boil, then reduce heat to medium-low. Simmer 1-3 minutes until slightly thickened (see note 2).
Remove pan from heat. Stir in Parmesan a small handful at a time until melted and mostly smooth. Add drained pasta and 1/2 cup reserved pasta water. Return pan to medium heat and toss with tongs until the sauce coats the pasta, about 1 minute. Add more pasta water if needed (see note 3).
Stir in sliced chicken. Taste and adjust salt and pepper. Remove from heat and serve right away. Garnish with parsley and an extra sprinkle of Parm if desired.
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Notes
Note 1: Parmesan is key to the flavor. Don’t use canned or pre-grated cheese; it won’t melt smoothly or taste right. Use a block of cheese and grate it finely with a zester or microplane. For an exceptional Alfredo, consider using Parmigiano Reggiano.Note 2: The sauce should coat the back of a spoon. Draw a line across it with your finger. If the line stays clear, it’s ready. If not, simmer a bit longer.Note 3: The sauce should be silky and cling to the pasta. If it gets too thick or grainy, stir in a splash of pasta water to smooth it out.Storage: This dish is best right after cooking. Store leftovers in the fridge and reheat gently with a splash of cream or chicken broth to loosen the sauce. The pasta will soften. Not recommended for freezing.