Preheat electric sandwich press or contact grill. In small bowl pour water over tomatoes. Let stand for 5 minutes. Drain well. Finely chop. Set aside.
Meanwhile, cook MorningStar Farms Spicy Black Bean burger according to package directions.
In another small bowl stir together tomatoes, mayonnaise, cocktail sauce and horseradish.
Lay out the bread slices onto a clean work surface. Spray one side of each slice with nonstick cooking spray. Flip 4 of the slices over and spread with the mayonnaise mixture. Place the burgers on top of the mayonnaise mixture then top each with 1 tablespoon of the pepper rings, 1 tablespoon of onion and remaining bread piece sprayed side facing up.
Place sandwich in press or grill. Cover and cook for 3 to 6 minutes or until bread is toasted. Slice in half and serve.