Prep the ingredients. Rinse and dry the cucumbers. Using a mandoline or a sharp knife, carefully slice the cucumbers into thin rounds. Thinly slice the onion. If using fresh, mince the fresh dill weed.
Make the dressing by placing the mayonnaise, vinegar, dill weed, lemon juice and the remaining ingredients in a large bowl. Gently whisk until dressing is smooth and well-combined.
Add the sliced cucumbers and onion to the bowl and toss well. Marinate in the fridge for at least 5 minutes before serving (the longer the better).