To make the mash, put the potatoes in a pan of cold water, season with salt and bring to the boil, then simmer for 10-15 minutes until tender. Drain then mash with a knob of butter, a splash of milk and the turmeric.
To make the filling, cook the lamb in a dry, non-stick frying pan until brown. Scoop out the pan, and pour away most of the fat.
Cook the onion, garlic and ginger in the same pan with a pinch of salt for about 5 minutes until softened. Add the spices and browned lamb mince, and cook for 1 minute, then stir in the flour and cook for another minute until it disappears.
Stir in the tomato purée and stock and simmer for 10-15 minutes until thickened. Stir in the frozen peas and coriander.
Heat the oven to 200C/fan 180C/gas 6. Tip the lamb mix into a baking dish and top with the mash. Fork to roughen the surface and dot with butter. Cook in the oven for 25-30 minutes until bubbling and golden on top.