Slice cucumbers into thin rounds or half-moons. Place in a large bowl.
Thinly slice red onion into rings. Place in a small bowl and cover with white vinegar. Let sit for 10 minutes.
Chop fresh dill and parsley finely.
Drain onions (reserve vinegar). Add onions to cucumbers along with herbs.
In a small jar, combine 3 tablespoons reserved vinegar, olive oil, salt, pepper, and sugar if using. Shake well.
Pour dressing over salad and toss gently. Taste and adjust seasoning. Serve immediately or refrigerate for 30 minutes for best flavor.