Heat the butter in a large skillet over medium heat. Add the diced onion and minced garlic. Cook for about 2 minutes until the onion softens.
Add the fresh spinach to the skillet and stir until it wilts, about 2-3 minutes.
Lower the heat. Stir in the heavy cream and cream cheese. Mix well until the cream cheese melts and the sauce is smooth.
Mix in the grated Parmesan cheese. Season with kosher salt. Let the mixture simmer for 2-3 minutes until the sauce thickens. If needed, add a splash more of heavy cream to thin it out.
Remove the skillet from the heat and serve immediately. Sprinkle with freshly ground black pepper.