Season chicken with salt, pepper, and Italian seasoning. Slice large breasts in half if needed for even cooking.
Cook rotini in well-salted water until al dente. Reserve 1/2 cup pasta water, then drain.
Heat olive oil and 1 tablespoon butter in a skillet over medium-high heat. Sear chicken 4–5 minutes per side until golden and cooked through. Remove and set aside.
Melt remaining butter in the same skillet. Add garlic and cook about 30 seconds until fragrant.
Pour in chicken broth and simmer while scraping up browned bits. Reduce heat and stir in heavy cream, Parmesan, and red pepper flakes (if using) until smooth.
Add reserved pasta water a little at a time if the sauce is too thick.
Return chicken to the skillet and spoon sauce over it. Add cooked rotini and toss gently to coat.
Simmer for 2 minutes, garnish with fresh parsley, and serve hot.
