Heat grill. Tear bread into chunks, and place in a food processor. Pulse to make fresh bread crumbs. Transfer crumbs to bowl; set aside. Add chicken, milk, onion powder, salt, allspice, and thyme to processor, and pulse just until combined. Add bread crumbs, and pulse to combine. Shape into 4 patties.
In bowl, combine onion, vinegar, sugar, and salt. Cook in microwave on high for 1 minute. Let cool 30 seconds. Stir in mango and cilantro. Spray patties with vegetable cooking spray, and grill 4 minutes on each side or until cooked through. Grill sourdough bread. Serve relish with burgers.