Peel the potatoes and let them cook for about 12 minutes. Drain and process into a smooth puree. Let cool down.
Cut bread into cubes. Using a fork, stir in the bread cubes, flour, semolina, baking powder, ½ teaspoon salt and the chopped herbs. Put aside.
In the meantime, cut the leek into thin, halved slices and the zucchini, put both in a saucepan with 1 tablespoon of oil and 2 tablespoons of water. Cover and simmer for about 5 minutes. Remove from heat and let cool down a little.
Fold the lukewarm leek and zucchini mixture into the dumpling dough, fluff with a little water if necessary. Shape into 6 dumplings with wet hands.
Let the water boil - place the dumplings in a steamer, cover and steam over the liquid over a gentle heat. If required, in two courses of 3 dumplings each. The dumplings take about 14 minutes.
Meanwhile, cut the onion into small pieces and sauté in 1 teaspoon oil until translucent - pour in the tomatoes, bring to the boil briefly, season with salt & pepper - add basil, chives & parsley.
First put the tomato sauce on a plate & garnish with 1 teaspoon of yoghurt in the soup dish and serve the dumplings.