Chop the onions and apples and fry them in 2 tablespoon butter. Then add the (frozen) peas and vegetable stock and simmer. You can vary the amount of vegetable broth between 500 and 700 ml or (if everything is already in) add hot water if it is too thick for you.
Meanwhile, if desired and available, peel the boiled potatoes, chop them up and add them to the simmering soup. After about 20 minutes, season the soup with salt, pepper, 2-3 pinches of sugar and the balsamic vinegar and work with a magic wand or put in the blender. Done - bon appetite!