In a small bowl, mix together the onion and 1 tablespoon of the salt. Set aside for 20 minutes. Rinse the onions 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.
In a small bowl, mix together the mayonnaise, sour cream, horseradish, zest, and 2 teaspoons salt. Season generously with pepper and hot sauce to taste. Refrigerate the horseradish sauce for at least 30 minutes.
Preheat the broiler. Slice the rolls in half lengthwise. Using your hands, shallowly scoop out the inside of each half. Transfer the rolls to a baking sheet and arrange scooped-side up. Toast the rolls under the broiler for about 1 minute. Slather the inside of the rolls with the horseradish sauce. On each of the roll 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 and cover with the tops of the roll. Serve immediately.