1Heat gas or charcoal grill. Brush both sides of sweet onion slices with oil; sprinkle with salt and pepper.
2Place onions on grill over medium heat. Cook uncovered, turning occasionally, until charred and softened. Remove onions from grill to cutting board; coarsely chop.
3In medium bowl, mix sweet onions, green onions, jalapeño, oil, vinegar and cumin. Season with salt and pepper. Serve relish at room temperature or chilled.