INGREDIENTS
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1 pound flank steak
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¼ cup + 2 tablespoons olive oil
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Salt and freshly ground black pepper, to taste
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1 medium red onion, thinly sliced
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½ cup apple cider vinegar
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8 slices Italian bread or 4 country rolls, split in half
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½ cup mayonnaise
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¼ cup prepared horseradish
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2 cups arugula, lightly packed