Preheat the oven to 425 degrees F.
Melt the butter in a large saute pan over medium heat. Add the onions and saute until translucent, about 15 minutes. Add the garlic and sauté on low heat for about 2 minutes, taking care not to burn the garlic. Add the flour and nutmeg and cook, stirring, for 2 more minutes. Add the cream and milk and cook until thickened. Squeeze or press the liquid from the spinach and add the spinach to the sauce. Add 1/2 cup of the Parmesan cheese and mix well. Season with salt and pepper, to taste.
Transfer the spinach mixture to a baking dish and sprinkle the remaining 1/2 cup Parmesan and the Gruyere on top. Bake for 20 minutes until hot and bubbly. Serve hot.
Chef’s Tip: This dish can be made ahead of time, the day before and refrigerated until ready to bake. Cover the spinach mixture.