Place mashed beans in medium bowl. Mix in next 6 ingredients. Shape into four 1/2-inch-thick patties. Prepare barbecue (medium heat). Spray cut sides of rolls and both sides of burgers and onion slices with nonstick spray. Grill cut sides of rolls until toasted, about 1 minute. Transfer rolls to plates. Grill burgers about 4 minutes per side for medium; grill onion slices about 5 minutes per side. Place 1 burger, 1 onion slice and some watercress on each roll bottom. Cover each with roll top.