Peel the potatoes, carrots, onions and radish, cut everything into large cubes. Heat the butter in a large saucepan, add the vegetables and ham and cook until the onions are translucent. Then deglaze with the vegetable stock. Bring to the boil and then cook on a low setting (preferably with the lid on) until the potatoes and carrots are soft. Then puree the vegetables finely and stir in the yogurt. Season everything with salt and pepper.