Heat the oil in a heavy pan and brown the lamb mince well until caramelised and dark at the edges
Add the onion and carrots and cook for ten minutes until softened
Stir in the garlic, thyme, rosemary and tomato paste, and cook for another minute
Pour in the stout and Worcestershire sauce, season lightly, and simmer until thick and glossy
Stir through the peas and set aside
Bake the potatoes whole at 200°C until soft, about one hour
Halve them while hot and scoop out the flesh
Pass through a ricer or sieve into a bowl, then beat in the butter, warm milk and salt until smooth and fluffy
Spoon the lamb mixture into an oven dish and scatter over the diced raw onion
Pipe or spoon the mash over the top, smoothing or ridging it as you like
Bake at 190°C for 25–30 minutes until golden
Scatter with chopped chives and serve hot
