Melt the butter in a pan, add the onions, cover and sweat until soft. Add the flour, stir and cook until brown. Add the stock, bring to the boil. Add the tomato puree, Ballymaloe Country Relish, herbs, salt and pepper. Simmer for about 5 mins.
Add the lamb and stir will. Transfer the mixture to a pie dish. Cover with mashed potato and score with a fork. Reheat in a moderate oven, 180ºC/350ºF/gas 4, for about 30 mins. Serve with some garlic butter and Ballymaloe Country Relish.