Preheat broiler. In a 2-cup microwave-safe glass measure or small bowl place dried tomatoes; add enough water to cover. Microwave, uncovered, on 100 percent power (high) for 1 minute. Let stand for 15 minutes.
Meanwhile, brush oil on the red onion slices. Trim fat from meat. Sprinkle meat with garlic salt and pepper. Arrange meat and onion slices on the unheated rack of a broiler pan. Broil 3 to 4 inches from the heat for 12 to 16 minutes for medium rare (145 degrees F) or desired doneness, turning once halfway through broiling. Transfer meat to a cutting board. Thinly slice meat across the grain into bite-size strips.
For sauce, drain and finely chop tomatoes. In a small bowl combine tomatoes, the 1/2 cup chopped onion, the mayonnaise, and mustard.
To serve, top bagel bottoms with salad greens. Top with meat, onion slices, sauce, and bagel tops.