Preheat broiler. Stem and halve green and red jalapeños; discard seeds. Set chiles cut side down in a shallow rimmed baking pan. Broil 4 in. from heat until chile skins are mostly charred, 10 to 15 minutes. Set chiles aside until cool enough to handle; coarsely chop.
In a large frying pan over high heat, combine red onion and oil. Stir often until onion softens slightly, about 5 minutes. Remove pan from heat and stir in jalapeños. Use warm.