1Bring a small pot of water to a boil over high heat. Add the onions and blanch for about 15 seconds. Drain well and set aside.
2Combine the lime juice, orange juice, sugar, and salt in a large, nonreactive bowl and stir until the sugar has dissolved. Add the blanched onions and the jalapeño and stir to coat thoroughly. Cover and refrigerate for at least 1 hour before serving.