Peel the potatoes, cover and cook in boiling salted water at low heat for about 20 minutes. Wash the chicken, dab dry and cut into wider strips. Clean, wash and cut the leek into narrow rings.
Heat the oil in a large frying pan. Fry the chicken meat for about 5 minutes until golden brown, season with salt and pepper, remove from the pan and put aside. Fry the leek in the frying fat, deglaze with the chunky tomatoes and stock, bring to the boil and let it thicken slightly for about 5 minutes.
Add the chicken and 1 teaspoon of thyme to the sauce, bring to the boil and season to taste. Drain the potatoes and serve with the chicken ragout. Serve sprinkled with remaining thyme.