Place a wide skillet over medium heat. Add butter and let it melt. Add chopped onion and cook until soft and pale, around 3–4 minutes. Stir in garlic and cook for 30 seconds until fragrant.

Sprinkle flour over the onion mixture. Stir gently for one minute so the raw flour taste cooks out. This step helps thicken the spinach cream.

Pour in half and half slowly, stirring the mixture the whole time. The sauce will start to thicken after a few minutes. Add salt, black pepper, and nutmeg.

Add fresh spinach in batches. Stir as it wilts down. Keep cooking until all the spinach softens and mixes into the sauce.

Lower the heat. Add cream cheese, mozzarella, and Parmesan. Stir until the mixture turns smooth and creamy. Taste and adjust seasoning if needed.

Remove from heat once the texture looks rich and spoonable. Let it rest for a minute before serving.