Wash and peel the zucchini, cut in half lengthways, remove the stones. Cut the zucchini into 1 cm cubes. Peel and wash the potatoes and also cut them into cubes. Clean the vegetable onion and cut into half rings.
Heat the butter in the saucepan, sauté the vegetables in it, fill up with the broth. Cover and simmer for 20 minutes over a mild heat. Cut the wieners into slices and let them get hot in the ragout for the last 5 minutes.
Mix the flour with about 6 tablespoons of cold water, add to the ragout and bring to the boil. Season the ragout well with salt, mustard and pepper. Mix the egg yolks with the cream and add them to the ragout.