Place 5 ounces spinach in a large colander in the sink. Bring a large saucepan of water to a boil over high heat. Add 8 ounces spaghetti and cook according to package directions. Drain the pasta over the spinach; toss to wilt the spinach.

Meanwhile, heat ½ cup sun-dried tomatoes along with 1 tablespoon of the oil in a large skillet over medium heat. Add ½ cup onion and sun-dried tomatoes; cook, stirring, until softened, about 3 minutes. Add garlic, ¼ teaspoon crushed red pepper, ¼ teaspoon salt and ¼ teaspoon pepper; cook, stirring, for 1 minute. Increase heat to medium-high and add 1 cup broth; cook, stirring, until reduced by about half, about 2 minutes. Stir in ½ cup sour cream, ¼ cup Parmesan and 1 tablespoon butter. Add the spaghetti and spinach; toss to coat well.