Slice cucumbers into thin rounds or half-moons. Toss with 1 teaspoon salt in a colander and let sit for 10 minutes.
In a medium bowl, whisk together yogurt, lemon juice, minced garlic, olive oil, dill, and mint until smooth.
After 10 minutes, gently squeeze the cucumber slices to remove excess liquid (do not rinse). Pat dry with paper towels.
Add cucumbers and sliced red onion to the dressing. Toss gently to coat.
Season with black pepper and a pinch of sugar if desired. Serve immediately or refrigerate for 30 minutes to let flavors meld.