Season the vegetables: In a large glass bowl (clear if you have it, so you can see the colors), add the sliced cucumbers, chopped tomatoes, and sliced onion. Sprinkle with about 1/2 teaspoon of the kosher salt, the black pepper, granulated garlic, dried oregano, and dried basil. Gently toss with clean hands or a large spoon to distribute the seasoning without crushing the tomatoes.
Make the marinade: In a separate measuring cup or small bowl, whisk together the white wine vinegar, red wine vinegar, lemon juice (if using), remaining 1/2 teaspoon kosher salt, and sugar until the sugar and salt start to dissolve. Slowly drizzle in the olive oil and neutral oil while whisking, until the mixture looks slightly thickened and well combined. Taste the marinade— it should be lightly tangy and a bit sweet; adjust with a pinch more sugar or a splash more vinegar to suit your family’s preferences.
Combine and toss: Pour the marinade evenly over the cucumber, tomato, and onion mixture in the glass bowl. Use a large spoon to gently toss everything together until the vegetables are well coated. Try to keep the tomatoes on top as you stir so they don’t get smashed; a folding motion from the bottom of the bowl works best.
Marinate: Cover the bowl tightly with plastic wrap or a lid. Let the salad sit at room temperature for 20–30 minutes to jump-start the marinating process, then transfer it to the refrigerator for at least 1 hour and up to 4 hours. During this time, the cucumbers will release some of their juices, which blend with the dressing to create a flavorful, drinkable marinade that even salad skeptics tend to love.
Finish and serve: Just before serving, uncover the bowl, give the salad a gentle stir, and taste the marinade. If it needs a little more brightness, add a small splash of vinegar; if it’s too sharp for your crew, stir in a teaspoon of sugar at a time until it tastes balanced. Sprinkle the chopped fresh parsley over the top for color. Serve the salad cold, making sure to spoon some of the marinade over each portion—this is often the part that converts the salad-haters.
