Thaw frozen spinach completely, then squeeze out every bit of extra water using a clean kitchen towel for better texture.

Melt butter over medium heat, then cook onion until soft before adding garlic and stirring for another minute until fragrant.

Whisk flour into the butter mixture, then slowly add milk and heavy cream while stirring until smooth and slightly thickened.

Mix in cream cheese and Parmesan until fully melted, creating a rich, creamy sauce without any lumps.

Fold spinach into the sauce, season with salt, pepper, and nutmeg, then simmer for several minutes before serving warm.