Lemon Sauce
Whisk all ingredients together.
Salad
Place the tomatoes in boiling water for 20 seconds. Remove and peel. Slice each tomato in half, and gently squeeze to remove most of the seeds. Chop into a 1.25 centimeter / 1/2 inch dice.
In a small bowl toss the onion slices with the salt. Gently squeeze the onion, then allow to rest for 20 minutes in the salt.
Rinse the salt off the onions in fresh water and squeeze to expel the remaining water. Layer, first the onions on a serving platter, then the tomatoes, then the mint. Pour lemon sauce over all. Serve at once.