Meanwhile, heat the oil from the sun-dried tomatoes in a large skillet over medium heat. Add the onion and sun-dried tomatoes, cooking and stirring until softened, about 3 minutes. Add the garlic, crushed red pepper, salt, and pepper, and cook for 1 minute, stirring constantly. Increase the heat to medium-high and add the broth, cooking and stirring until the liquid reduces by about half, about 2 minutes. Stir in the sour cream, Parmesan, and butter. Add the cooked spaghetti and wilted spinach, tossing to coat everything evenly.