Slice cucumbers into thin rounds, about 1/8-inch thick. If using English cucumbers, leave skin on; for waxed cucumbers, peel first. Thinly slice red onion into half-rings.
In a large bowl, whisk together sour cream, lemon juice, vinegar, sugar, salt, and pepper until smooth. Taste and adjust seasoning if needed.
Add cucumber slices, onion, and chopped dill to the bowl. Toss gently until evenly coated with dressing.
Cover bowl and refrigerate for at least 30 minutes before serving to allow flavors to meld.
Stir again before serving. Garnish with extra dill if desired.