1. Preheat the oven to 350°F.
2. In a pan, heat the butter. Add the onion and cook for 2 minutes. Add the spinach, garlic, nutmeg, salt, and pepper; cook for 2 minutes.
3. Plunge the mussels into a pot of boiling water for 50 seconds to open them. Drain. Open them, keeping the mussel in one side of the shell.
4. Place a bit of spinach on top of each mussel.
5. Sprinkle with parmesan and place on a baking sheet.
6. Bake to make the parmesan crust, about 4 minutes.
7. Remove from the oven and serve immediately.