1. Prepare the Spinach:

• If using fresh spinach, wash the leaves thoroughly and remove any tough stems. In a large pot, bring about 1-2 cups of water to a boil, and then add the spinach in batches, stirring until wilted. Once the spinach has wilted, drain it in a colander and squeeze out any excess water.
• If using frozen spinach, make sure to thaw it thoroughly and drain out any excess water.

2. Make the Creamy Base:

• In a large skillet or saucepan, melt the butter over medium heat. Add the chopped onion and cook for about 3-4 minutes, or until softened and translucent.
• Add the minced garlic and cook for another minute, stirring constantly, until fragrant.
• Stir in the flour to make a roux. Cook the flour for 1-2 minutes, stirring constantly, until it turns a light golden color. This will help thicken the sauce later.

3. Add the Dairy:

• Gradually whisk in the milk and heavy cream, making sure to whisk constantly to prevent lumps from forming. Continue to cook over medium heat until the sauce begins to thicken, about 3-5 minutes.
• Stir in the cream cheese (if using) and allow it to melt into the sauce for extra richness.

4. Season the Cream Sauce:

• Add the ground nutmeg, white pepper, and salt. Taste and adjust the seasoning as needed.

5. Combine with Spinach:

• Add the wilted or drained spinach to the sauce, stirring to combine. Cook for about 2-3 minutes, allowing the spinach to fully heat through and absorb the flavors of the creamy sauce.
• Stir in the grated Parmesan cheese and cheddar cheese (if using), allowing the cheeses to melt and blend into the sauce, making it extra creamy and flavorful.

6. Serve:

• Once the spinach is thoroughly mixed with the sauce and cheese, transfer it to a serving dish.
• Optionally, garnish with a bit more Parmesan cheese or freshly ground black pepper for a finishing touch.