Roast Beef Sandwiches

Name: Roast Beef Sandwiches
Ingredients: 1 medium red onion, halved and thinly sliced; 1 tablespoon, plus 2 teaspoons kosher salt, plus more to taste; 6 tablespoons red wine vinegar; 3/4 cup mayonnaise; 3/4 cup sour cream; 1/4 cup plus 2 tablespoons jarred grated horseradish (with liquid); 1/2 teaspoon grated lemon zest; Freshly ground black pepper; Hot sauce; 6 Kaiser rolls; 12 slices vine-ripened tomatoes; 24 ounces freshly sliced rare roast beef; 3 loosely packed cups watercress or arugula, or a combination of both