Cook the pasta. Bring a large pot of salted water to a boil and cook the pasta until al dente.
Reserve about 1 cup of pasta water, then drain the rest and set the pasta aside.
Season the chicken. Pat the chicken dry and season it with salt, pepper, and garlic powder. This helps build flavor from the start.
Brown the chicken. Heat olive oil in a large skillet over medium-high heat. Add the chicken in a single layer and cook until golden and cooked through, about 5 to 7 minutes.
Remove it from the pan and set it aside.
Make the sauce base. Lower the heat to medium and add the butter to the same skillet. Stir in the garlic, sun dried tomatoes, tomato paste, Italian seasoning, and red pepper flakes if using. Cook for about 1 minute until fragrant.
Add the liquid ingredients. Pour in the chicken broth and scrape up any browned bits from the bottom of the pan.
Stir in the heavy cream and let the sauce simmer gently for 3 to 4 minutes.
Finish the sauce. Add the Parmesan cheese and stir until it melts and the sauce thickens slightly. If the sauce looks too thick, add a splash of the reserved pasta water.
Bring everything together. Add the cooked chicken and spinach back to the skillet. Stir until the spinach wilts and the chicken warms through.
Toss with pasta. Add the drained pasta and stir until it is fully coated in the sauce.
Taste and adjust with more salt, pepper, or a little pasta water if needed.
Serve right away. Top with fresh basil and extra Parmesan if you want. This dish tastes best when served warm and creamy.