Bring 2quarts of lightly salted water to a boil in a covered pot over high heat. Add the string beans, cover, and cook for 2 minutes. Add the corn, cover, and continue cooking for another minute. Test the string beans; they should be crisp-tender, but if you prefer them more tender, cook for another minute. Drain the string beans and corn, then turn onto a platter to cool the vegetables to room temperature.
Meanwhile, place the dressing ingredients in a blender cup. Cover and blend for about 30-60 seconds, until well blended, stopping once to scrape down the sides of the blender cup with a rubber spatula. Taste the dressing for seasoning.
After the vegetables have cooled slightly, turn into a bowl. Add the tomatoes and their juices, and the onion. Toss gently to combine, using two wooden spoons. Pour the dressing evenly over the vegetables, using a rubber spatula to scrape the blender cup. Using two wooden spoons, toss the salad gently but thoroughly until well coated. Taste for seasonings.