Preheat the oven to 200°C. In a pan, cook the potatoes in boiling, salted water for twenty minutes, drain and mash together with 15ml of the olive oil, set aside.
In a large pan, heat the oil over a moderate heat, add the onions and sauté until soft. Add the mince, thyme, all the vegetables and the tomato puree and sauté until the mince is brown.
In another pan, braise the celery heart in a little chicken stock for ten minutes. Add the rest of stock and simmer for for twenty minutes unitl tender.
Line the base of a pie dish with a layer of the mashed potato. Add the lamb mince and vegetables and top the mince with another layer of mashed potato. Arrange a mix of celery hearts and chopped stalks on the top and bake in the oven for twenty minutes, serve immediately.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.