Preheat the oven to 425 degrees F. Melt the butter in a heavy-bottomed sauté pan on medium heat. Add the onions and cook for about 5 to 6 minutes or until translucent. Add the spinach to the sauce, cook for about a minute or until the spinach is just wilted.

Add the flour and nutmeg, stir and cook for 2 more minutes. Add the cream and milk and cook until thickened. Add 1/2 of the Parmesan cheese and mix well. Season with salt and pepper and transfer to a baking dish and sprinkle the remaining Parmesan and the Gruyere. Bake for 20 minutes until hot and bubbly. Serve immediately.