In the same skillet, melt the butter. Add the onion, sun-dried tomatoes, and tomato paste. Cook for 2 to 3 minutes, until the onion softens. Stir in the garlic and cook for 1 minute.
Pour in the chicken stock and scrape up the browned bits. Add the heavy cream, Parmesan, crushed red pepper, kosher salt, and black pepper. Lower the heat to medium-low and bring the sauce to a gentle simmer.
Stir in the spinach and cook until wilted, about 3 to 4 minutes.
Add the linguine and toss until the noodles are coated. Top with the chicken and serve right away.