2) Rinse the onion with cold running water. Drain and squeeze to remove excess liquid. Combine the onions and the vinegar and marinate at least 30 minutes or up to 24 hours.
3) In a small bowl, mix together the mayonnaise, sour cream, horseradish, zest and 2 tsp salt. Season generously with pepper and hot sauce to taste. Refrigerate the horseradish sauce for at least 30 minutes.
4) Preheat the grill. Slice the baps in half lengthwise. Using your hands, shallowly scoop out the inside of each half. Transfer the baps to a baking sheet and arrange scooped-side up. Toast the rolls under the grill for about 1 minute. Slather the inside of the baps with the horseradish sauce.
5) On each of the bap bottoms, layer 2 tomato slices and season with salt and pepper to taste. Top the tomatoes with some of the roast beef overlapped into "ruffles" and season with salt and pepper. Top the meat with some of the onions and watercress or rocket and cover with the tops of the bap. Serve immediately.
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