Preheat Oven To 375 Degrees F (190 Degrees C). Set An Oven Rack Into The Center Of The Oven. Line A Baking Sheet With Aluminum Foil And A Layer Of Cooking Spray. Arrange Halibut Pieces On The Prepared Baking Sheet So That All Pieces Are Touching. Mix The Mayonnaise, Lemon Juice, Chili Garlic Sauce, Sugar, Soy Sauce, And Sesame Oil In A Bowl; Fold The Mushrooms And Onion Into The Mayonnaise Mixture. Spread The Mushroom Mixture Over The Halibut Pieces. Squeeze One Lemon Half Over The Mushroom Mixture; Sprinkle With Kosher Salt. Bake In The Preheated Oven Until Almost Opaque, 15 To 20 Minutes; Remove The Baking Sheet From The Oven. Raise The Oven Rack With The Baking Sheet To The Next-Highest Level Switch The Oven To Broil. Continue Cooking The Fish Under The Broiler Until The Top Is Browned, About 10 Minutes. Squeeze The Remaining Lemon Half Evenly Over The Fish. Sprinkle With Sesame Seeds And Green Onions.