Put the oil in a large frying pan on a medium heat and wait until it is properly hot (you can tell by holding your hand about 15cm above the pan). When hot, add the fennel, cumin, black mustard, fenugreek and nigella seeds, and leave them to sizzle for 30 seconds to a minute, but no longer. Add the runner beans and corn kernels, fry, stirring, for three or four minutes, then add the garlic and onion, and fry for two minutes more.