Soak the onion in a bowl of ice water for at least 15 minutes. Drain and pat dry. Core and slice the tomatoes about 1/3" thick. Sprinkle each side lightly with salt (preferably kosher) and let drain on paper towels for about 10 minutes. (Alternatively, you could catch the tomato juices by draining them in a colander over a bowl and save the juices for a latter use). On each of the four plates, arrange overlapping slices of tomato. Sprinkle onion and mint over the tomatoes. Decoratively add a drizzle of olive oil. Finish with some fresh ground pepper.