Step 1Fill a large bowl with salted ice water. Peel the cucumbers and slice them 1/8-to 1/4-inch thick. Place the cucumber slices in the water and let them stand 1 hour.
Step 2Meanwhile, combine the sour cream, vinegar, sugar, dill and celery seeds, mixing well. Refrigerate until ready to use.
Step 3Drain the cucumbers well and let them stand in a colander to air-dry, or lay the slices on paper towels. Place the slices in a serving bowl, add the onion and gently toss with the sour cream mixture.
Step 4Chill thoroughly, at least 2 hours, before serving.