First of all wipe the mushrooms with some damp kitchen paper if they need it. Melt the butter in a medium-sized saucepan and soften the onion in it over a medium heat for about 5 minutes. Then chop the mushrooms fairly small and add them to the pan along with the sherry. Stir well, season with salt and pepper and a little grated nutmeg, then lower the heat, put a lid on and leave for 5 minutes for the heat to draw out the mushroom juice. Next add the milk, stock and mushroom ketchup, stir and simmer again with the lid on for 10 minutes. After that transfer half the soup to a blender and whiz to a purée, then return it to the saucepan along with the lemon juice. Taste to check the seasoning, then reheat gently, adding the cream just before serving. Garnish with slices of raw mushroom.