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Make the dressing by whisking the vinegar, oil, onion seeds and spring onions together, then set aside. Slice the tomatoes, then arrange over a large serving platter and lightly season with salt. Drizzle over most of the oil and set aside.
Slice the steak and arrange the tomatoes over it. Drizzle with any resting juices. Scatter over the artichokes and charred onions, then the feta and chives. To serve, drizzle over the dressing.