How to Make It
Preheat broiler to high.
Heat a grill pan over medium-high heat. Coat pan with cooking spray. Arrange onion slices on pan; cook 2 minutes on each side or until lightly charred. Remove pan from heat; coarsely chop onions.
Combine chopped onions, pesto, oil, salt, pepper, and chicken in a medium bowl; toss well. Divide mixture evenly among mushrooms, pressing gently to fill each cap.
Return pan to medium-high. Coat with cooking spray. Gently transfer mushrooms to pan; cook 3 to 4 minutes or until mushrooms are heated through and tender. Sprinkle cheese evenly over chicken mixture. Transfer pan to oven; broil 2 minutes or until cheese melts and begins to brown.