Wilt 4 cups fresh spinach: Place the spinach in a colander and pour boiling water over it to wilt, then set it aside.
Cook 8 oz whole-wheat spaghetti: In a large pot of salted boiling water, add the spaghetti and cook according to package instructions until al dente. Drain it over the wilted spinach in the colander to allow the heat to wilt the spinach further, then set aside.
Prepare the sauce: In a large skillet, heat a splash of reserved oil from the sun-dried tomatoes (if available) over medium heat. Sauté 1 small diced red onion and 1/2 cup chopped sun-dried tomatoes for about 3-4 minutes until the onion becomes translucent. Add 2 cloves minced garlic, 1/2 tsp crushed red pepper flakes, and salt & pepper to taste. Sauté for another minute until fragrant.
Add the broth: Increase the heat to medium-high and pour in 1 cup vegetable or chicken broth. Allow it to simmer until reduced by half, about 5-7 minutes.
Make it creamy: Lower the heat to medium again, then mix in 1/2 cup sour cream, 1/2 cup grated Parmesan, and 2 tbsp butter. Stir until well combined and the mixture becomes creamy.
Toss the pasta: Add the drained spaghetti and wilted spinach to the skillet, tossing everything together until the pasta is coated with the sauce and heated through, approximately 2-3 minutes.