Wash the potatoes well and cook in boiling water for about 20 minutes. Wash and clean the mangetouts. Cook the sugar snap peas and peas in boiling salted water for about 5 minutes. Drain and rinse with cold water.
Drain the potatoes, rinse with cold water and peel. Leave to cool. Mix mustard, vinegar, 2 tablespoons of oil, yoghurt and stock. Season with salt and pepper. Mix with the potatoes and leave to stand for about 30 minutes.
Clean, wash and quarter the radishes. Wash the meat, dab dry and cut into strips. Heat 2 tablespoons of oil in a frying pan, fry the meat for about 3 minutes while turning, season with salt and pepper.
Mix radishes, peas, mangetout and chicken strips into the salad. Season to taste with sugar, salt and pepper. Garnish with chervil.