Peel the carrots and potatoes. Cut everything into small pieces, the faster it will be done. Chop the onion and set everything aside.
Melt the butter in a large saucepan and sweat the onion in it. Put the chopped carrots and potatoes in the pot and fill with water. So high that everything is covered with water. Salt the water a little. Bring the water to a boil and let everything boil until the water has almost disappeared. Make sure that you have to stir more often towards the end and that the hob is a little smaller so that nothing burns. If the water has boiled off, remove the pan from the stove.
Mash everything in the pot with a potato masher so that it becomes a beautiful, fine vegetable. Then season with salt, pepper and mustard.
Finally, bury the crispy sausages in the vegetables so that they get warm.