Peel and finely dice the onion and garlic. Peel the potatoes and cut into 3 cm pieces. Heat the oil in a large pan and fry the mince until crumbly. Sauté the onion and garlic briefly. Stir in the spices and tomato purée, fry briefly, then pour in the hot soup and add the tomatoes. Add the potatoes, cover and simmer for approx. 20 minutes until the potatoes are al dente. Coarsely crumble the feta, add to the pan and stir in. Flavour with salt and pepper. Serve garnished with parsley and crème fraiche.