Bring a large pot of salted water to a boil. Add pasta and cook until al dente, about 8–10 minutes. Reserve 1 cup of pasta water before draining.

In a large skillet over medium heat, melt the butter. Add the diced chicken, season with salt and pepper, and cook until golden brown and cooked through, about 6–8 minutes. Remove and set aside.

In the same skillet, add more butter if needed. Sauté minced garlic and diced onion until translucent and fragrant, about 3–4 minutes.

Pour in the heavy cream and chicken broth. Stir in Italian seasoning and red pepper flakes. Allow the mixture to simmer for about 5 minutes until slightly thickened, adjusting with reserved pasta water if necessary.

Return the cooked chicken to the skillet and add the drained pasta, tossing well to combine, cooking for an additional 2 minutes.

Serve hot, topped with grated Parmesan and freshly chopped parsley.