Wash the spinach very well under cold water. Remove thick stems if needed. Bring a large pot of water to boil. Add spinach and cook for one minute only. Drain it quickly and rinse with cold water. Squeeze out all extra water. Chop the spinach roughly and set aside.

Place a large skillet on medium heat. Add the butter and let it melt slowly. Do not let it brown. Keep the heat gentle. The butter creates the rich base for the sauce. I always love this step because the kitchen starts smelling warm and inviting.

Add chopped onion to the melted butter. Cook for about three minutes until soft and slightly golden. Stir often to prevent burning. Add minced garlic and cook for one more minute. The aroma becomes strong and delicious at this stage.

Sprinkle flour over the onion mixture. Stir continuously for two minutes. This step removes the raw flour taste. The mixture will look thick and slightly pasty. Cooking the flour properly is important for smooth sauce texture.

Slowly pour in the milk while whisking constantly. This prevents lumps from forming. Once smooth, add heavy cream. Continue stirring until the sauce thickens. The mixture should feel creamy and coat the spoon nicely.

Add salt, black pepper and ground nutmeg. Nutmeg gives that classic steakhouse flavor. Stir well and taste the sauce. Adjust seasoning if needed. The flavor should feel rich and slightly savory.

Mix in grated Parmesan cheese first. Let it melt completely. Then add shredded mozzarella. Stir gently until smooth and creamy. The sauce will now look thick and glossy.

Add the chopped spinach to the cream sauce. Stir until every piece is coated. Cook on low heat for five minutes. The mixture should not be watery. If too thick, add a small splash of milk.

Taste one last time before serving. Adjust salt or pepper if needed. Serve hot beside steak or roasted chicken. This is exactly how to make House of Prime Rib Creamed Spinach at home with easy steps.