To eliminate the unpleasant sharpness of onions, Chilean cooks thinly slice them and then soak them in salted or sugared water.
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In a medium bowl, toss the onion slices with the sugar and cover with cold water. Let soak for 10 minutes. Drain and rinse the onion, then pat dry.
Layer the tomato and onion slices on a large platter or in a bowl; season generously with salt and drizzle with the olive oil. Sprinkle with the jalapeños and cilantro and serve.