Bring a large pot of well-salted water to a rolling boil. Cook pasta according to package directions until al dente, about 8 to 10 minutes. Before draining, scoop out 1 cup of pasta water and set aside. Drain pasta and set aside.

Melt 2 tablespoons of butter in a large skillet over medium heat. Add chicken pieces, season with salt and pepper, and cook for 6 to 8 minutes, stirring occasionally, until golden brown and cooked through with no pink remaining. Remove chicken from skillet and set aside.

In the same skillet, melt remaining 2 tablespoons of butter over medium heat. Add minced garlic and diced onion. Cook for 3 to 4 minutes until soft and translucent.

Pour in heavy cream and chicken broth, stirring to combine. Add Italian seasoning and red pepper flakes. Simmer on medium-low for about 5 minutes until sauce thickens and coats the back of a spoon. Use reserved pasta water to adjust consistency if needed.

Return cooked chicken and drained pasta to the skillet. Toss to coat fully. Cook for another 2 minutes to meld flavors.

Remove from heat. Sprinkle freshly grated Parmesan over the top and let melt for 30 seconds. Taste and adjust seasoning. Garnish with fresh parsley and serve warm.