Bring a large pot of salted water to a boil. Cook pasta according to package directions until al dente. Reserve 1 cup of pasta water, then drain.
While pasta cooks, season the chicken breasts with Italian seasoning, salt, and pepper. Heat olive oil in a large skillet over medium-high heat. Cook the chicken for 5-7 minutes per side, until golden and cooked through. Remove from skillet and set aside.
In the same skillet, add the chopped onion and cook for 2-3 minutes until soft. Add the minced garlic and cook for 1 minute more until fragrant.
Pour in the chicken broth to deglaze the pan, scraping up any browned bits. Bring to a simmer.
Reduce heat to medium-low. Stir in the heavy cream and sun-dried tomatoes. Simmer gently for 3-4 minutes to thicken. Stir in the grated Parmesan cheese until the sauce is smooth and creamy.
Slice the rested chicken. Add the cooked pasta, sliced chicken, and fresh spinach to the skillet. Stir until the spinach wilts and everything is coated in the sauce. If needed, add a splash of reserved pasta water to reach the desired consistency.
Serve immediately, garnished with extra Parmesan if desired.
