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
Combine onions, vinegar, and sugar in medium bowl; sprinkle with salt and pepper.
Let stand until onions are soft, tossing occasionally, at least 1 hour and up to 4 hours.
Place bottom halves of rolls on work surface. Sprinkle with cheese.
Top each with 3 tomato slices, then 1/4 of roast beef.
Top each with 1/2 cup watercress, then 1/4 of pickled onions. Cover with tops of rolls.
Cut each sandwich in half.