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

Meanwhile, heat sun-dried tomatoes along with oil in a large skillet over medium heat. Add onion and sun-dried tomatoes; cook, stirring, until softened, about 3 minutes. Add garlic, crushed red pepper, salt and pepper; cook, stirring, for 1 minute.

Increase heat to medium-high and add broth; cook, stirring, until reduced by about half, about 2 minutes. Stir in sour cream, Parmesan and butter. Add the spaghetti and spinach; toss to coat well.