Sauté the aromatics. In a large pan over medium heat, melt the butter and add the chopped onion. Sauté for about 3-4 minutes until the onion is soft and translucent. Add the minced garlic and cook for another 30 seconds until fragrant.

Cook the spinach. If using fresh spinach, add it to the pan in batches, letting it wilt down before adding more. If using frozen spinach, make sure it’s well-drained and add it directly to the pan.

Make the creamy sauce. Pour in the heavy cream and milk, stirring to combine. Sprinkle in a pinch of nutmeg, salt, and pepper. Let it simmer for a couple of minutes until slightly thickened.

Add the cheese (if using). Stir in Parmesan or Gruyère until melted and incorporated into the sauce. This step adds extra depth and flavor.

Final touch. Taste and adjust seasoning as needed. If the sauce is too thick, add a splash of milk. If it’s too thin, let it cook for another minute.

Serve warm. Transfer to a serving dish and enjoy alongside your favorite main course!