Put the biltong in a food processor and blitz to a powder as fine as you can make it. Tip into a bowl and set aside. Scatter the onion in a dry saucepan and cook over a low heat for about 30 mins until very dark and soft, adding a little oil if needed to stop it burning. Meanwhile, heat the 2 tbsp oil in a wide pan and crumble in the mince. Cook on a low heat for 30 mins – it will release lots of liquid, which needs to evaporate, then eventually it will sizzle in its own fat. If the pan becomes too dry, drizzle in a little more oil. Towards the end of the cooking time, stir the mince until it becomes speckled with crisp brown bits. Mix in the cumin, coriander and chipotle paste, then stir in the stock, bring to a simmer and cook for 10 mins.