Place the fresh spinach leaves in a large colander set in the sink.

Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package directions. Once cooked, drain the pasta over the spinach in the colander to wilt the spinach. Set aside.

In a large skillet, heat 1 tablespoon of the reserved oil from the sun-dried tomatoes over medium heat. Add the chopped onion and sun-dried tomatoes; cook until softened, about 3 minutes. Stir in the minced garlic, crushed red pepper flakes, salt, and pepper; cook for another minute.

Increase the heat to medium-high and add the broth to the skillet. Cook until the liquid is reduced by half, about 2 minutes. Stir in the sour cream, Parmesan cheese, and butter until well combined and creamy.

Add the cooked spaghetti and wilted spinach to the skillet. Toss everything together until the pasta is evenly coated with the sauce.