1. Start by thawing the frozen spinach and squeezing out as much liquid as possible. If using fresh spinach, sauté it in a skillet until wilted, then drain well and chop it before proceeding with the recipe.

2. Melt the butter in a skillet over medium heat. Add the onion and cook for 3 to 5 minutes, stirring occasionally, until softened. Then, add the minced garlic and cook for an additional 30 seconds, being careful not to let it burn.

3. Sprinkle the all-purpose flour over the onion and garlic mixture and stir for 1 minute to cook off the raw flour taste. Slowly whisk in the half-and-half. Add the nutmeg, salt, and pepper, then let the mixture simmer for 3 to 5 minutes, stirring occasionally, until thickened.

4. Stir the prepared spinach into the cream sauce until it is evenly coated and heated through. Add the grated Parmesan cheese and the cream cheese (if using), and continue stirring until the cheese has melted and the mixture is creamy and smooth.

5. Taste the Steakhouse-Style Creamed Spinach and adjust the seasoning as needed. Serve the dish warm, allowing the flavors to meld together for a truly indulgent experience.