If you don't have time to allow the onions to sweat, you can take a spoon and sort of crunch them up to release some of their natural flavors. I often use this approach. Simply mince the onions, put them in the jar, put the seasonings on top, and crunch/mash slightly with a spoon. Ideally, you will let them sit at this point for a couple minutes while you cut up the salad, then add the vinegar and oils.