Preheat broiler. Place tomatoes in small bowl; cover with water. Microcook on 100 percent power (high) for 1 minute. Brush steak and onion slices with oil; sprinkle with salt and pepper. Arrange on unheated rack of broiler pan. Broil, 3 to 4 inches from heat, 12 to 16 minutes or until desired doneness, turning once. Thinly slice beef across grain in bite-size strips.
Drain tomatoes. Spread mayonnaise on bagels; fill with sliced beef and onions, tomatoes, and greens. Serves 4.