Wash the beans thoroughy and string them from both sides. Cut them into 1/4 inch pieces. Peel and cube the potatoes into pretty much the same size as the broad beans.
Heat oil in a broad based pan/kadai and toss in the cumin seeds. When the stop spluttering add the chopped onions and fry them till translucent (pink).
Now reduce the heat completely and add the powders, give it a quick stir and add the potatoes and stir for a few seconds. Add in the broad beans and salt to taste. Cover and cook on sim till the potatoes are done. Do not add any water, the vegetables will cook in the steam.
Make sure to stir the mixture every now and then so that the veggies don't get scorched. Serve hot with rice or chapathis