Place spinach in a colander in the sink. Cook spaghetti in a large pot until al dente (8‑10 minutes), then drain it over the spinach to wilt it with the hot pasta.

Meanwhile, heat 1 Tbsp of the reserved sun‑dried tomato oil in a large skillet over medium heat. Add onion and sun‑dried tomatoes; sauté for about 3 minutes until softened.

Stir in garlic, crushed red pepper flakes, salt and pepper; cook another minute.

Increase heat to medium‑high and pour in the broth; simmer until reduced by half (~2 minutes).

Stir in sour cream, Parmesan cheese and butter until the sauce is smooth and creamy.

Add the spaghetti and wilted spinach to the skillet, tossing to coat evenly with the sauce.

Serve hot, garnished with extra Parmesan or a squeeze of lemon juice and fresh herbs if desired.