In a small saucepan, bring about 2 cups of water to a boil.
Remove the pan from the heat and add the sliced onion.
Soak the onion for 1 minute.
Drain and transfer the onion to a small bowl.
Add the vinegar, 2 tablespoons water, salt, and sugar.
Stir and let stand for 20 to 30 minutes.
Meanwhile, arrange the sliced tomatoes on a serving plate.
Drain the onions and scatter over the tomatoes.
Sprinkle with cilantro or oregano.