Combine first 5 ingredients in a food processor; process until finely ground. Divide chicken mixture into 4 equal portions; shape each into a 1/2-inch-thick patty. Heat a large grill pan over medium-high heat. Coat pan with cooking spray. Add patties to pan; cook 5 minutes on each side.
Place bottom half of each bun on a plate, and spread 1 tablespoon mayonnaise over each. Top each burger with 1 lettuce leaf, 1 patty, and 2 tomato slices. Spread 1 1/2 teaspoons mustard over each bun top, then top burgers.