Bring a large pot of salted water to a rolling boil and add pasta. Cook according to package directions, then drain and set aside. Reserve 1 cup pasta water for sauce consistency adjustments throughout cooking.
While pasta cooks, heat olive oil in a large skillet over medium-high heat. Season chicken pieces with salt and pepper, then cook for 6-7 minutes until golden on all sides. Transfer chicken to a clean plate temporarily.
In the same skillet, melt butter and sauté diced onion for 3-4 minutes until softened and fragrant. Add minced garlic and cook for exactly one minute, stirring constantly to prevent burning.
Pour in chicken broth and heavy cream, stirring gently to combine all components evenly. Return cooked chicken to the pan and add Italian seasoning, red pepper flakes, and sun-dried tomatoes. Simmer for 8-10 minutes until sauce thickens slightly and chicken cooks through completely.
Stir in grated parmesan cheese until fully melted and smooth, creating that signature creamy pasta sauce. Fold in fresh spinach and cook for just 2-3 minutes until wilted but still bright green.
Add cooked pasta to the sauce and toss gently, adding reserved pasta water if needed for desired consistency. Taste and adjust seasonings with salt and pepper as needed before plating.