INGREDIENTS
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1 small red onion, halved lengthwise, thinly sliced crosswise (about 1 cup)
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3 tablespoons red-wine vinegar
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6 ripe medium tomatoes, cored and cut into wedges
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1 English cucumber, partially peeled lengthwise to create stripes, then cut crosswise into 1/2-inch-thick slices, larger pieces halved
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2 to 3 tablespoons extra-virgin olive oil
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1/4 cup packed fresh basil, larger leaves torn
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Sea salt and freshly ground pepper