In a saucepan, melt the butter and fry the chicken for 2–3 minutes. Add the onions and continue to cook for another 3–4 minutes. Add the garlic, chicken stock and seasoning. Bring to the simmer and cook for 15–20 minutes until the chicken is tender. While the chicken is cooking, plunge the leeks into salted, boiling water and cook for 2–3 minutes until tender. Drain and leave to cool.