Boil the pasta: Bring a large pot of salted water to a boil.
Cook pasta until just shy of al dente. Reserve 1/2 cup pasta water, then drain.
Season the chicken: Pat chicken dry. Toss with salt, pepper, and Italian seasoning.
Sear the chicken: Heat olive oil in a large skillet over medium-high. Add chicken in an even layer and cook 4–6 minutes, stirring once or twice, until golden and cooked through.
Transfer to a bowl.
Build the garlic base: Reduce heat to medium. Add butter to the skillet. When melted, add garlic and cook 30–60 seconds until fragrant.
Don’t brown it.
Thicken slightly: Sprinkle in the flour and stir for 30 seconds to form a paste.
Add liquids: Whisk in chicken broth until smooth. Simmer 1–2 minutes, then stir in cream. Cook another 2–3 minutes until lightly thickened.
Finish the sauce: Stir in Parmesan until melted.
Add a pinch of red pepper flakes if using. Season with additional salt and pepper to taste.
Bring it together: Return chicken and any juices to the pan. Add drained pasta and toss to coat, loosening with reserved pasta water as needed for a silky sauce.
Brighten and serve: Stir in lemon juice.
Top with parsley and more Parmesan. Serve hot.