Heat olive oil in a large flameproof casserole dish, add lamb and brown all over. Transfer to a plate.
Reduce the heat under casserole, add vegetables then sauté for 10 minutes, stirring frequently.
Remove casserole from the heat, add meat, sprinkle in flour and mix well. Pour in enough hot water to cover meat and vegetables, stir well and return to the heat.
Preheat oven to Gas Mark 4, 180°C, 350°F.
Bring casserole to the boil, stirring often as gravy thickens. Season and add Worcestershire sauce. Remove from heat.
Slice potatoes thinly by hand or with a mandolin. Layer them carefully to completely cover meat and vegetables.
Place in the oven and cook for 2 hours. The potatoes should be golden on top and the gravy bubbling up around sides.
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.
Taken from Fran Warde's cookbook Real Food For Families
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.