Put a large colander in a large bowl. Add the cucumbers and onions to the colander, sprinkle with 1 tablespoon salt and toss until the salt is evenly distributed. Let stand at room temperature for at least 1 hour and up to 2 hours.
Transfer the cucumbers and onions from the colander to a paper towel-lined baking sheet and pat them dry with another layer of paper towels.
Whisk together the sour cream, dill, vinegar and a few cracks of black pepper in a medium bowl. Add the cucumbers and onions and toss until coated.
To make ahead: Salt and pat dry the cucumbers and onions as above, then keep separate from the dressing until ready to serve.
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