Cut the zucchini, potatoes and onions into small pieces and place in a saucepan with the broth and salt. Fill up with 400 ml of water and bring to the boil, then simmer for about 15 - 20 minutes, then puree.
In the meantime, prepare the couscous with a little broth in another pot according to the instructions and then add a small piece of butter.
Distribute everything on 4 plates and sprinkle with the grated cheese.