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 1/2 cup sun-dried tomatoes along with 1 tablespoon of the oil in a large skillet over medium heat. Add 1/2 cup onion and sun-dried tomatoes; cook, stirring, until softened, about 3 minutes.

Add garlic, 1/4 teaspoon crushed red pepper, 1/4 teaspoon salt, and 1/4 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 1/2 cup sour cream, 1/4 cup Parmesan, and 1 tablespoon butter. Add the spaghetti and spinach; toss to coat well.