Cut the chicken breasts into 1-inch pieces; pat them dry with paper towels and season evenly with salt and black pepper. Bring a large pot of salted water to a rolling boil. Add the fettuccine and cook according to package directions until al dente. Reserve "0.5" cup of the pasta cooking water before draining.

Heat "1" tbsp of olive oil in a large skillet or Dutch oven over medium-high heat. Add the seasoned chicken pieces and cook for 5-7 minutes, stirring occasionally, until they are golden brown on all sides and cooked through. Remove the chicken from the skillet and set aside.

Reduce the heat to medium. Add the minced garlic to the same skillet and cook for 1 minute until fragrant, being careful not to burn it. If the pan seems too dry, add a tiny splash more olive oil.

Pour in the chicken broth and scrape up any browned bits from the bottom of the pan with a wooden spoon. Bring to a simmer, then stir in the heavy cream and fresh lemon juice. Let it gently simmer for 3-5 minutes, allowing the sauce to thicken slightly.

Remove the skillet from the heat. Stir in the "0.5" cup of grated Parmesan cheese and the chopped fresh parsley until the cheese is melted and the sauce is smooth. If using, stir in the red pepper flakes. Taste and adjust seasoning if needed.

Add the cooked pasta and the seared chicken back into the skillet with the sauce. Toss gently to coat everything evenly. If the sauce seems too thick, add a tablespoon or two of the reserved pasta water until it reaches your desired consistency.

Divide the pasta among serving plates. Garnish with additional grated Parmesan cheese, fresh lemon zest, and more chopped parsley, if desired. Serve immediately.