In a small bowl, mix together the red wine vinegar (2 tablespoons), sugar (1 teaspoon), kosher salt (1/2 teaspoon), and black pepper (1/4 teaspoon). Add the sliced red onions and stir to coat. Allow to sit for 5-10 minutes, stirring once or twice.
While you let the onions sit: cut the ends off the cucumber, slice in half lengthwise, and scoop out the seeds with a spoon. Discard the seeds and slice the cucumber thinly, about 1/4" thick. Add to a large bowl. (And if you prefer, you can peel the cucumber before cutting it.)
Cut the cherry tomatoes in half and add to the large bowl as well.
If your feta cheese isn't already crumbled, use a fork to crumble it and add it to the large bowl.
Drizzle the olive oil (1/4 cup) into the small bowl with the onions, add the chopped fresh parsley (2 tablespoons) or other herbs, and mix together.
Pour the onion mixture into the large bowl with the remaining ingredients and toss to coat. Serve immediately for best results.
To make this ahead of time: do everything EXCEPT mixing the onion mixture with the cucumbers, tomatoes, and feta. Just before serving, toss everything together (otherwise it will get soggy, since the salt will draw the moisture out of the cucumbers).
Time saving tip: I highly recommend buying a block of feta cheese and crumbling it yourself- it's more flavorful. But if you're short on time, pre-crumbled will do just fine!
Money saving tip: I used cucumbers from my garden for this (as well as tomatoes). English cucumbers are more expensive than regular cucumbers, but they have fewer seeds and a less bitter peel. If you're looking to save money, buy regular cucumbers and peel them if you prefer.
Variations: Try adding rotisserie chicken or chickpeas to make this a complete meal, and/or grains such as cooked and cooled pearl couscous or orzo. Add other popular Mediterranean ingredients, like kalamata olives, capers, or roasted red peppers, to add more flavor.
The provided nutrition information does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Feel free to calculate it yourself using this calculator or by adding the recipe to Yummly.