Heat the oil in a flameproof casserole. Dry the meat with kitchen paper (wet meat doesn’t brown well) and brown it all over, working in batches. Remove each batch to a bowl as it’s ready.
Add the onion to the pan and cook until soft and golden, stirring frequently. Add the garlic and chilli and cook for another 2 minutes.
Stir in the cumin and cook for 2 more minutes. Return the lamb to the pot – along with any juices that have run out of it – plus the tomatoes, lager, tomato purée, sugar and oregano. Season really well and bring to just under the boil.
Immediately reduce the heat to a very gentle simmer and cook, uncovered, for 2 hours or until the lamb is completely tender. Keep an eye on the amount of liquid and stir every so often. Add the beans halfway through. You want the finished chilli to be thick.
Add the lime juice, spring onions and coriander and taste for seasoning, adjusting it if you need to (you might want to tweak the amount of sugar, as well as salt, pepper or lime juice).
Serve with rice, soured cream or yogurt, avocado slices tossed with lime juice, and grated cheese.