Wash the leek thoroughly and cut the light green and white parts into delicate rings.
Cut the chicken breast fillets into strips. Peel the onion and cut into rings. Wash, clean and grate the carrots. Clean the mushrooms and cut into thin slices.
Cut the tomatoes crosswise at the top, hold them briefly in boiling water, peel, halve and core and cut the flesh into strips. Mix everything together loosely.
Mix the mayonnaise and yoghurt and season with mustard, salt, pepper and a little vinegar. Pour the marinade over the salad.