Wash cucumbers and tomatoes thoroughly. Slice cucumbers into rounds or half-moons, and dice the tomatoes into bite-sized pieces.
Thinly slice red onion and add it to the cucumbers and tomatoes.
Sprinkle a pinch of salt and let the vegetables rest for 10 minutes to draw out moisture.
Drizzle olive oil and lemon juice (or vinegar) over the vegetables.
Toss everything lightly to coat the vegetables without crushing the tomatoes.
Add black pepper and fresh herbs if desired. Taste and adjust seasoning. Serve immediately or store dressing separately if preparing ahead.