First of all, know that this recipe is extremely flexible. Change it up and make it your own! Use garlic or not, use red onions, yellow onions, sweet onions, green onions, shallots, or none of those. Use whichever herbs sound good to you: dill is a classic, but so is oregano. And remember, you never have to make it the same way twice.
Peel and slice the red onion very thinly. Taste it – if it’s too hot for your taste, soak the slices in a bowl of ice-water and store in the fridge for about an hour, they’ll be milder then. Chop the tomatoes roughly; you should have about 2 cups. Peel the cucumbers and halve them lengthwise, and then use a spoon to scoop out the seeds and gel. Slice them crosswise in chunks.
In a large bowl, mix the oil and vinegar well, then add garlic and herbs. Put the tomatoes and cucumbers (and onion, when it’s to your liking) in the bowl, toss well to combine. Let this sit in the refrigerator so that the flavors meld – at least an hour.