Preheat the oven to 375°F (190°C) and grease a 9x13 inch baking dish.

In a large pot, bring salted water to a rolling boil and cook the elbow macaroni according to the package instructions until al dente, about 7-8 minutes. Drain and set aside.

In the same pot, melt unsalted butter over medium heat, then add finely chopped onion and minced garlic. Sauté until the onion is translucent, about 3-4 minutes.

Stir in the all-purpose flour and cook for 1-2 minutes to form a roux. Gradually whisk in the whole milk and heavy cream, stirring until thickened (about 5-7 minutes).

Add ground nutmeg and black pepper to the sauce, mixing well.

Fold in the spinach, cheddar cheese, and Parmesan cheese until melted and smooth. If using mustard powder, add it now.

Mix the cooked macaroni into the cheese sauce until well combined.

Spread the mixture in the greased baking dish and sprinkle with panko breadcrumbs, if desired. Bake for 25-30 minutes until golden brown.

Remove from the oven and let cool for 5 minutes before serving.