Roast Beef Sandwiches with Sweet-and-Sour Red Onions and Blue Cheese

Name: Roast Beef Sandwiches with Sweet-and-Sour Red Onions and Blue Cheese
Yield: 4 Servings
Ingredients: 2 cups thinly sliced red onions; 2 tablespoons red wine vinegar; 1 teaspoon sugar; 4 large soft whole grain rolls, toasted; 1/2 cup crumbled blue cheese; 12 tomato slices; 8 ounces thinly sliced roast beef; 2 cups (packed) small watercress sprigs