Slice cucumbers thinly (use a mandoline if you have one) and place in a colander. Toss with 1 teaspoon salt and let sit for 10 minutes.
In a medium bowl, whisk together sour cream, vinegar, lemon juice, sugar, dill, and pepper until smooth.
After 10 minutes, gently squeeze handfuls of cucumber slices to remove excess liquid (do not rinse). Pat dry with paper towels.
Add cucumbers and red onion (if using) to the dressing and toss gently to coat.
Cover and refrigerate for at least 30 minutes to let flavors meld. Garnish with extra dill before serving.