Preheat the oven to 425 degrees. Spray or butter a 9x13 baking dish.

On the stovetop over medium heat, melt the butter in a saute pan. Add the chopped onions and saute until translucent, about 10 to 15 minutes. Add the garlic and saute for an additional 2 minutes. Add the flour and nutmeg. Stir in and cook for 2 additional minutes. Add the cream and milk. Cook until thickened, about 3 to 5 minutes.

Squeeze the liquid from the spinach. (Be sure to squeeze as much liquid from the spinach as you can so the dish does not get runny). Add the spinach to the onions and cream mixture. Stir to combine.

Add 1/2 cup of the Parmesan cheese and stir to fully incorporate. Season with the salt and pepper to taste.

Transfer the spinach mixture to the prepared 9x13 baking dish. Sprinkle the remaining 1/2 cup Parmesan cheese and Gruyere cheese over the top.

Bake fo 20 minutes or until hot and bubbly and the cheese has melted on top.