1. Peel the vegetables and chop them coarsely; sauté in butter for a few minutes. Season with salt and pepper. 
2. Add the chicken stock and cook until the vegetables are tender, about 10-15 minutes. 
3. Purée everything in a blender with the milk and 200 g (7 oz.) crab.
4. Reheat;  correct the seasoning. 
5. Serve hot, adding a spoonful of sherry to each bowl. 

Garnish

1. At the last minute, whir the cress and olive oil in the blender until very foamy.
2. Place a small spoonful of crab on the side of the soup; top with a drizzle of cress mousse.