Cook the Pasta
Bring a large pot of salted water to a boil and cook spaghetti until al dente according to package instructions.
Reserve 1 cup of pasta water before draining.
Sauté the Onions
Heat olive oil in a large skillet over medium heat.
Add the finely chopped onion and sauté until translucent, about 5 minutes.
Add Garlic and Sun-Dried Tomatoes
Stir in minced garlic and cook for about 1 minute until fragrant.
Add chopped sun-dried tomatoes and cook for 2 minutes to release their flavor.
Prepare the Cream Sauce
Pour in the heavy cream and reduce heat to low.
Simmer for 3-5 minutes, allowing the sauce to thicken slightly.
Incorporate Parmesan and Spinach
Stir in grated Parmesan cheese until fully melted and the sauce is smooth.
Add fresh spinach and stir gently until it wilts but remains bright green.
Combine Pasta and Sauce
Toss the cooked spaghetti into the skillet, mixing until the pasta is evenly coated with sauce.
Use reserved pasta water to adjust the sauce's consistency if needed.
Season and Serve
Season with salt, pepper, and optional red pepper flakes to taste.
Serve hot, garnished with extra Parmesan cheese if desired.