2 ounces (55g) red onion (1/4 medium 8-ounce onion), thinly sliced pole to pole
1/4 cup (60ml) red wine vinegar
12 ounces (340g) mixed ripe tomatoes, cut into slices and chunks (about 2 heaping cups when cut up)
12 ounces cucumber (340g; about 2 small cucumbers), peeled (or partially peeled or unpeeled, if you want some of the bitter skin); quartered lengthwise; and thinly sliced crosswise
1/4 cup pitted Kalamata or other briny black olives (about 1 1/2 ounces; 40g), see note
1/2 cup (120ml) extra-virgin olive oil, plus more for drizzling
Kosher or sea salt
2 big pinches dried Greek or Mediterranean oregano, divided
4 ounces (115g) feta cheese, preferably cut into slabs
In a small bowl, combine onion with vinegar and let soak while you prepare the other ingredients, about 15 minutes. Drain onions, reserving vinegar.
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In a salad bowl or large mixing bowl, combine tomatoes, cucumber, olives, onion, olive oil, and about 2 tablespoons (30ml) of the vinegar left over from quick-pickling onion. Season with salt and one large pinch of oregano, toss gently to combine, then adjust to taste with more salt and vinegar, if desired.
Lay slabs of feta on top, sprinkle with remaining pinch of oregano, and drizzle with olive oil. Serve, soaking up juices with bread.