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, then drain and set aside.

In a large skillet, melt 2 tablespoons of butter over medium heat. Add diced chicken seasoned with salt and pepper, and cook for 6-8 minutes until golden brown and cooked through. Transfer chicken to a plate.

In the same skillet, melt remaining butter, then add minced garlic and chopped onion. Sauté for 3-4 minutes until onion is translucent.

Pour in heavy cream and chicken broth, stirring to combine. Add Italian seasoning and red pepper flakes, then bring to a gentle simmer for 5 minutes, adjusting with reserved pasta water as needed.

Add the sautéed chicken and drained pasta to the skillet, tossing gently until well-coated. Cook for an additional 2 minutes over low heat.

Remove from heat, sprinkle Parmesan cheese on top, allowing it to melt. Garnish with fresh parsley and serve warm.