Wash potatoes thoroughly and cook in boiling water for about 20 minutes. Drain, rinse with cold water and peel. Cut potatoes into slices and pour the stock over them. Leave to stand for about 30 minutes.
In the meantime, boil the egg hard for 10 minutes. Roast the sunflower seeds in a pan without fat until golden brown, remove. Cut the cucumber into slices. Clean and wash spring onion and cut into rings.
Clean, wash and cut the salad into bite-sized pieces. Drain the potatoes and catch the broth. Mix stock, oil and vinegar. Season with salt and pepper. Peel and halve the egg. Put one half back as a snack.
Mix the potato slices, cucumber and spring onion with the vinaigrette. Arrange on a plate with the radicchio and the egg half. Sprinkle with sunflower seeds.