Heat the oil in a medium-size saucepan over medium-high heat. Add the mustard seeds, cover the pan, and cook until the seeds have stopped popping (not unlike popcorn), about 30 seconds. Reduce the heat to medium-low and add the bell peppers and chiles. Roast, uncovered, stirring occasionally, until the peppers sweat, their juices evaporate, their skin acquires blotchy brown