Brush fish evenly with 2 tablespoons pesto; sprinkle with salt and pepper. Brush 1 tablespoon pesto over onion slices. Toss tomatoes with remaining 1 tablespoon pesto, and place on a 12-inch square of heavy-duty foil. Fold edges of foil up around tomatoes to form a bowl, keeping tomatoes in a single layer. (Do not completely enclose.)
Place salmon, onion slices, and foil bowl with tomatoes on a grill rack coated with cooking spray. Grill 14 minutes or until salmon is desired degree of doneness, onion is tender, and tomatoes begin to burst, turning salmon and onion after 7 minutes.
Using a fork, gently break salmon into large chunks, and chop onion slices. Combine Orzo Salad with Arugula and White Beans, salmon, onion, tomatoes, and accumulated tomato juice. Toss gently; sprinkle with pepper.