Combine the onion and two vinegars in a medium bowl and set aside for at least 30 minutes and up to 2 hours.
Wash the cucumbers well and slice them thinly using a sharp knife or a mandoline. Toss the cucumber with several large pinches of salt in a sieve or colander set over a bowl to catch the juices. Set aside for about 30 minutes, tossing occasionally.
Whisk the caraway seeds, vegetable oil and salt and pepper to taste into the onion vinegar mixture.
Spread the cucumber slices on a double layer of paper towels and dry them well. No more than 30 minutes before you are going to serve the salad, toss the cucumbers with the dressing and the dill. Taste and add more salt and pepper if necessary.