Halve the avocado lengthways, remove the stone and take the pulp out of the skin. Cut the onion into small pieces.
Put the avocado and onion in a saucepan, add a few dashes of lemon juice and either puree everything to a juicy mass with a hand blender or mash it with a potato masher (if you like to have bits in the soup, the second variant is the better option) . Then add the vegetable stock, milk and cream, stir well and bring everything to a boil. Now season well with salt and pepper and add herbs as required (e.g. parsley).