Peel the onion and slice it into thin rings. If the flavor is too sharp, mix 1 teaspoon of red wine vinegar with 150 ml (about 2/3 cup) of water and soak the onion slices for 5-10 minutes. Then drain and gently pat them dry with a clean kitchen towel.
Meanwhile, wash and dry the tomatoes, cucumber, and bell pepper. Slice the tomatoes into thick wedges. If the cucumber has tough or bitter skin, peel it. Slice the cucumber into 0.5-1 cm (about 1/4–3/8 inch) rounds, or into half-moons if it’s large. Thinly slice the bell pepper into strips.
In a large bowl, combine the tomatoes, cucumber, onion, bell pepper, and olives. Season with salt to taste and toss gently.
Top the salad with a whole slab of feta, then drizzle with olive oil and sprinkle with dried oregano.
