Finely chop the onion, carrot, celery and garlic. Then place in a pan with the olive oil and sauté for 5 minutes over a medium heat.
Add the lamb mince and break up well with a wooden spoon; cook until it is browned. Sprinkle in the flour and mix well, add the tomato purée, stock, bay leaf, Worcestershire sauce and pepper, mix well, cover and simmer for 30 minutes, stirring from time to time.
Peel the potatoes and cut into even-sized pieces, cover with hot water and simmer for 20 minutes. Drain when cooked, return to the pan and shake over a low heat to steam off excess moisture.
Remove from the heat and crush with a potato masher until smooth, add hot milk, butter and white pepper and beat until fluffy and well blended.
Heat the oven to Gas 5, 190°C, fan170°C. Remove bay leaf from the meat and place the meat in an ovenproof dish. Carefully spoon the mashed potato onto the mix and use a fork to spread the potato over the top, sealing in the meat and making a stripey pattern.
Place in the oven and bake for 30 minutes until top of the potato is slightly golden. Serve with a green salad or steamed broccoli.
Try sprinkling some grated cheese evenly over the mashed potato for a change, or chop some parsley and sprinkle over the shepherd's pie before serving for an attractive finish.
Freezing and defrosting guidelines
Once the dish has cooled completely, transfer it to an airtight, freezer-safe container, seal and freeze for up to 1-3 months. To serve, defrost thoroughly in the fridge overnight before reheating. Loosely cover with foil and bake until dish is thoroughly heated through. Reheat until piping hot.
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.