Place 18 cleaned mussels in a pot with a splash of water. Cover tightly and cook over high heat until opened, 3-5 minutes. Take out of shells, reserving half the shells for serving. Heat 1 tablespoon butter with 2 cloves crushed garlic and allow to sizzle for about 1 minute without browning.
Add ½ cup cream and 200g baby spinach leaves and toss over heat until wilted. Season with salt, pepper and a pinch of nutmeg. Divide this mixture among 18 half shells (about 2 teaspoons per shell). Toss the mussels in the cream mixture left in the pot and then return each to a shell on top of spinach. (Mussels can be prepared ahead to this point and chilled until ready to serve.)
When ready to serve, sprinkle with a little parmesan and place under a hot grill for 3-4 minutes until bubbling and lightly golden. Serves 6 as finger food.