Heat the butter in a saucepan, add the onion and fry gently for 5 minutes until softened. Add the bacon and potato, toss in the butter then cover and fry for 5 minutes until just beginning to brown.
Pour on the stock, crumble in the chestnuts, then add the nutmeg and salt and pepper. Bring to the boil, then cover and simmer for 30 minutes. Add the sliced sprouts and mix into the stock, then cover and simmer for 5 minutes until they are tender but still bright green.
Allow the soup to cool slightly, reserve a few sprout slices for the garnish, then purée it in batches in a blender or food processor until green specks can still be seen. Pour back into the saucepan and reheat. Taste and adjust the seasoning if needed. Garnish with the remaining Brussels sprouts and bacon.