Heat the oil in a large, broad casserole dish over a high heat and
brown the minced lamb. You may need to do this in batches, as
putting too much meat in the pan will make it sweat instead of fry.
Transfer the browned lamb to a bowl.

In the same pan, cook the onions and carrots, until the onions are
pale gold and soft. Add the garlic and spices and cook for another
minute. Return the lamb to the pan, then add the stock or water,
orange zest and juice, tomato purée, sugar and oats. Season, bring
to the boil, then reduce the heat and cook, uncovered and stirring
from time to time, for 45 minutes tot hour: The stock should
reduce so that you have a good thick mixture. If it seems too dry,
add more stock or water.

Meanwhile, make your sweet potato topping. Preheat the oven to
190°C/375°F/gas mark 5. Set the sweet potatoes on a baking sheet
and bake them in their skins; they will take about 40 minutes to
become tender, depending on their size. When they’re soft, slit
them down the middle and scoop the flesh into a bowl. Mash it
with half the butter and some salt and pepper.

Spoon the lamb into a pie dish and spread the sweet potato
on top, roughing it up with a fork to give it an attractive, uneven
surface. Dot with the rest of the butter. Return to the oven and
cook for 20 minutes, or until golden and bubbling up at the
edges. Scatter with coriander or parsley before serving, if you like.