Preheat oven to 150 C / Gas 2. Generously grease a 22cm square baking dish with butter.
Spread the sliced Brussels sprouts out into the prepared baking dish in an even layer. In a bowl, whisk together the cream, Parmesan cheese, garlic, salt, and black pepper until thoroughly combined; pour the cream mixture over the Brussels sprouts. Sprinkle the top with nutmeg. Cover with aluminium foil.
Bake in the preheated oven for 1 hour; remove foil, and bake uncovered until the top has browned, 15 more minutes.