Split avocados in half with a knife and remove the pits. Set aside one half. Scoop out the insides of the other three halves with a spoon and blend with 1 cup of the stock in a blender until smooth. Stir in the curry powder, salt, pepper, cream, and remaining stock. Chill. When ready to serve, garnish the soup with thin avocado slices that have been cut from the remaining avocado half and dipped in lemon juice. Serves 4 to 6.