Place the cumin, coriander, fennel, mustard and cardamom in a mortar and crush with a pestle. Add the chilli flakes. 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, deep saucepan or flameproof casserole dish and fry the onion, capsicums and eggplant 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 about 30 minutes until the meat is tender. Serve scattered with toasted flaked almonds.