01 - If using fresh spinach, blanch in boiling water for 1 to 2 minutes until wilted. Drain, cool, squeeze out excess liquid, and roughly chop.
02 - In a large skillet over medium heat, melt butter. Add chopped onion and sauté for 2 to 3 minutes until translucent. Add minced garlic and cook for 30 seconds.
03 - If using flour, stir it into the skillet and cook for 1 minute, stirring constantly.
04 - Pour in heavy cream and bring to a gentle simmer. Add cream cheese and stir until melted and the sauce is smooth.
05 - Add the chopped spinach to the skillet and mix thoroughly to coat in the cream sauce.
06 - Stir in grated Parmesan and ground nutmeg. Season with salt and black pepper to taste. Simmer for 2 to 3 minutes until heated through and sauce has thickened to preference. Serve hot.