Preheat the Instant Pot pressure cooker on sauté mode and add ½ tablespoon of butter to melt. Pat the chicken dry with paper towel. Season with Kosher salt and freshly ground black pepper, and add the chicken to the pot in 2 batches (use the additional ½ tablespoon butter for the second batch), and cook undisturbed for 1 to 2 minutes on each side. Transfer to a plate, and set aside.
Add the garlic to the pot and cook until fragrant, about 1 minute. Be sure to stir so it doesn't burn. Add the chicken stock to the pot and scrape the pot with a wooden spatula to deglaze.
Break the fettuccine noodles in half and lay half of them flat in the pot, then top with the other half in the other direction. Add water if needed to cover the noodles. Add the chicken strips back to the pot, on top of the noodles.
Secure the lid and cook on high pressure for 3 minutes, then allow the pressure to naturally release for about 5 minutes. Open the vent and remove the lid. Press cancel and select sauté.
Use tongs to remove the chicken from the pot and set in a bowl, then set aside. Use the tongs to separate any noodles that may have clung together. Stir the remaining 1 tablespoon of butter, the heavy cream, and the Parmesan cheese into the fettuccine in the pot and cook until the sauce begins to thicken. Season with more salt and pepper to taste, and return the chicken to the pot, tossing in the alfredo sauce.
Serve with more Parmesan cheese and sprinkle with parsley.
Follow the same directions as the Instant Pot, but use a large, high-sided skillet for the stovetop. Cook the chicken for 5 minutes or until cooked through, and set aside. Cook the noodles in the chicken stock for 10 minutes or until al dente. Then, add the butter, cream, and Parmesan cheese and stir to combine. Add the reserved chicken, reduce the heat to medium, and cook until the sauce thickens. Adjust seasoning and serve with more Parmesan cheese and minced parsley.
