Cook the pasta separately. Bring a large pot of well-salted water to a boil and cook your pasta according to package instructions. Reserve about 1/2 cup of pasta water, then drain and set aside.
Cook chicken. Heat butter in a large skillet over medium heat. Add finely diced onion and sauté until soft and translucent, about 5 minutes. Season chicken with salt and pepper then add it to the skillet. Cook until it is lightly golden on the outside and fully cooked through, about 5 minutes per side. Add garlic and cook for 30 seconds.
Pour in the chicken broth, scraping up any browned bits from the pan. Let the sauce gently simmer for a few minutes to thicken slightly. Stir in sundried tomatoes, heavy cream, and red pepper flakes. Let simmer for 5 minutes.
Stir in baby spinach and parmesan cheese. Let parmesan cheese completely melt.
Add in cooked pasta and stir together. Add in some of the reserved pasta water if you desire a thinner sauce. Season with more salt and pepper if needed, optional Italian seasoning, and fresh basil.
Serve with extra parmesan and basil ribbons.
Notes
Heavy cream is recommended for the richest sauce, but half-and-half can work for a lighter option. Milk may make the sauce too thin. Store in the refrigerator in an airtight container for 3-5 days. Reheat gently on the stove or microwave, adding a splash of cream, milk, or chicken broth to loosen the sauce.
Keyword dinner recipe, marry me chicken pasta, marry me pasta