1) Preheat the oven to 210°C.
2) Melt the butter in a heavy-bottomed saute pan over medium heat. Add the onions and saute until translucent, about 15 minutes. Add the flour and nutmeg and cook, stirring, for 2 more minutes. Add the cream and milk and cook until thickened.
3) Squeeze as much liquid as possible from the spinach and add the spinach to the sauce. Add 50g of the parmesan cheese and mix well. Season, to taste, with salt and pepper.
4) Transfer the spinach to a baking dish and sprinkle the remaining parmesan and the gruyere on top. Bake for 20 minutes until hot and bubbly.
5) Serve hot.