Slice the cucumbers and red onion into thin rounds, about 1/8-inch thick.
Optional: If you prefer milder onions, place sliced red onions in a bowl of ice water for 10 minutes, then drain and pat dry.
In a small bowl, whisk together olive oil, white wine vinegar, salt, pepper, and sugar (if using).
Place the sliced cucumbers and onions in a large bowl.
Pour the dressing over the vegetables and toss gently to coat evenly.
Add fresh dill if using and give a final toss.
Taste and adjust seasonings as needed.
Serve immediately for maximum crispness or chill for 15 minutes to enhance flavors.