In a medium saucepan, fry the onion in the oil till it's limp. Add the curry leaves and the raisins, and cook another minute or two. Pile the potatoes on top, then sprinkle with salt and garam masala. Pour water over - enough so that there's about 1 inch / 2.5 cm of water in the pan. Cover the pan, bring to the boil, turn down to medium-low, and cook till the potatoes are thoroughly done - 15 or 20 minutes. Add the peas, stir it up, and serve.