To make about 1 1/2 pints of Roman mustard combine the mustard seed, vinegar and grape juice, and mix in the salt and cumin. Leave to soak, covered but not sealed for 36 to 48 hours. Put into a food processor and whiz for 1 to 2 minutes or until coarsely ground, then add the almonds and pine nuts and whiz very briefly until they are completely broken up, not over-processed.
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.