Place the cumin, coriander, fennel, mustard and cardamom in a mortar and crush with a pestle. Add the chillies. Place the lamb in a bowl, add the spices and toss to coat evenly. Cover and chill for about 2 hours, or overnight.
Heat the oil in a large pan or flameproof casserole and fry the onion, peppers and aubergine for about 10 minutes, stirring occasionally, to soften.
Add the spiced lamb and lentils, then stir in the stock and cinnamon. Bring to the boil, then stir, reduce the heat and cover. Simmer gently for 30–35 minutes until the meat is tender. Serve scattered with toasted almonds.
Curries improve in flavour if made ahead, allowing time for the flavours to mingle. Chill, then reheat gently until thoroughly heated through.