Top and tail the beans, then slice them into 1-inch pieces diagonally. Cook in boiling salted water for 5 to 10 minutes, depending on their age. They should be tender but still have a slight bite. Drain.
Heat 1 tablespoon of the oil in the empty pan. Blend the chili, garlic, and mustard together with a pinch of salt. Add to the pan and stir-fry for 30 seconds. Add the beans and stir thoroughly over a very low heat.
In a small pan heat the remaining oil, add the mustard seed and cumin, and cover. Cook until they start to splutter. Pour over the beans and serve immediately.