In the same skillet, over medium heat, melt the unsalted butter.
Add the minced garlic and finely chopped onion; sauté for 2 - 3 minutes until the onion is softened and the garlic is fragrant.
Pour in the heavy cream and chicken broth, stirring well to combine; bring the mixture to a simmer.
Stir in the grated Parmesan cheese until the sauce is smooth and thickened.
Add the chopped spinach and sun-dried tomatoes; cook for another 2 - 3 minutes until the spinach is wilted.
Season the sauce with salt and pepper, to taste.