Cook the pasta according to package directions until al dente. Place spinach in a colander in the sink and drain the pasta over it 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 red onion and sun-dried tomatoes, sautéing for about 3 minutes until the onion softens.

Stir in the garlic, red pepper flakes, salt, and pepper, cooking for 1 minute until fragrant. Add the broth and bring to a simmer, cooking until reduced by half, about 2 minutes.

Then add the sour cream, Parmesan, and butter, stirring until smooth. Add the pasta and wilted spinach to the skillet, tossing to coat.

Serve immediately, top with additional Parmesan if desired.