Prepare the Spinach:

• If using fresh spinach, blanch it in boiling salted water for 1-2 minutes, then transfer to an ice bath. Drain thoroughly, squeezing out excess water, and chop finely.
• If using frozen spinach, thaw completely and squeeze out any excess moisture.

Sauté the Onions and Garlic:

• Preheat the oven to 375°F (190°C).
• In a large saucepan, melt the butter over medium heat. Add the chopped onion and cook for 5-7 minutes, until softened. Add the garlic and cook for 1 minute.

Make the Béchamel Sauce:

• Sprinkle the flour over the onions and garlic, stirring constantly to create a roux. Cook for 1-2 minutes, then gradually whisk in the warm milk and cream. Continue whisking until the sauce thickens and is smooth about 3-5 minutes.
• Stir in the nutmeg, Parmesan, Gruyère, salt, and pepper. Mix until the cheese is melted and the sauce is creamy.

Combine with Spinach:

• Fold the chopped spinach into the béchamel sauce, ensuring it is evenly coated.

Assemble the Gratin:

• Transfer the spinach mixture to a greased 9×13-inch baking dish. Sprinkle breadcrumbs evenly over the top.

Bake the Gratin:

• Bake for 20-25 minutes, or until the topping is golden brown and the gratin is bubbling around the edges.

Serve Hot:

• Let the gratin cool for a few minutes before serving. Enjoy it as a side dish to roasted meats or fish.