Fill a large pot with 4-5 quarts of cold water and bring to a rolling boil over high heat. Salt generously and set a heat-proof measuring cup nearby for reserving pasta water.
Pat chicken breasts completely dry with paper towels. Slice each breast in half horizontally to create thinner cutlets. Combine Italian seasoning, garlic powder, onion powder, salt, and pepper in a small bowl and season both sides of the cutlets.
Heat olive oil in a large skillet over medium-high heat. Carefully place the seasoned chicken cutlets in the pan without overcrowding. Cook undisturbed for 4-5 minutes until golden and naturally releasing from the pan. Flip and cook another 3-4 minutes until internal temperature reaches 165°F/74°C. Transfer to a plate and tent with foil.
Add fettuccine to the boiling water and cook 1-2 minutes less than package directions for al dente. Before draining, reserve 1/2 cup of starchy pasta water. Drain the pasta but do not rinse.
Return the same skillet to medium heat. Add butter and let it melt, scraping up the browned bits from the chicken. Add minced garlic and cook 60-90 seconds until fragrant, stirring constantly to prevent burning.
Pour the Alfredo sauce into the skillet and stir into the garlic butter. Add heavy cream and stir until combined. Reduce heat to low and warm gently, stirring frequently, for 3-4 minutes without boiling. Gradually stir in the freshly grated Parmesan until smooth and glossy. Thin with reserved pasta water if needed.
Slice the rested chicken into strips or bite-sized pieces against the grain. Add drained fettuccine to the skillet and toss until fully coated in sauce. Arrange sliced chicken on top and give a final gentle toss to incorporate.
Divide between warmed bowls and finish with extra grated Parmesan, fresh parsley, and a crack of black pepper. Serve immediately while sauce is at its silkiest.