Rinse the cucumbers and tomatoes well and pat them dry.
Thinly slice the cucumbers into rounds about ⅛ inch thick.
Chop the tomatoes into bite-size pieces.
Peel the onion and slice it very thinly into half-moons.
Add the cucumbers, tomatoes, and onion to a large glass bowl.
Sprinkle with the kosher salt, black pepper, garlic powder, dried oregano, and dried basil.
Gently toss without crushing the tomatoes.
In a small bowl or measuring cup, whisk together the vinegar, lemon juice, sugar, and olive oil until combined.
Pour the marinade over the vegetables.
Gently fold everything together until the cucumbers, tomatoes, and onion are evenly coated.
Cover and refrigerate for at least 1 hour and up to 4 hours before serving.
Just before serving, stir gently and spoon some of the marinade over each portion.
Serve cold.
