Heat the oil in a heavy-based, medium-sized saucepan over medium heat until very hot. Add the mustard seeds and fenugreek seeds, and cook for 10 seconds until aromatic or the seeds stop crackling (keep a lid handy to stop seeds from flying out). Add the onion and stir-fry for 5 minutes. Add the curry powder, chili, kohlrabi and red lentils and stir-fry for another minute. A