Heat oil in a large pan over medium-high heat. Stir in mustard seeds. When mustard seeds begin to crack, stir in onion. Cook 1 minute, then stir in capsicums, turmeric, salt, and chili powder. Reduce heat to medium low, cover, and cook 3 to 4 minutes. Stir in besan, mixing well; cover, and cook 2 minutes more.