Wash the potatoes and cook in boiling water for about 20 minutes. Drain, rinse with cold water and remove the skin. Peel and finely chop the onion. Heat the oil in a pan and fry the onion.
Add 200 ml water, bring to the boil, stir in the stock. Remove from heat and season with salt, sugar and vinegar. Cut potatoes into slices and mix with the marinade. Cut Gouda into cubes. Wash, clean and drain the rocket and radicchio.
Cut radicchio into strips. Wash the apple, quarter it, remove the seeds and cut it into thin slices, mix everything into the salad. Season to taste with salt and pepper. Roughly chop the nuts. Wash and pluck the grapes.
Sprinkle both on the salad and serve.