1.In A Small Bowl, Combine Sliced Avocado And Lemon Juice. Add Water To Cover; Set Aside. Preheat An Outdoor Grill For High Heat And Lightly Oil Grate. 2.Heat Butter In A Large Heavy Skillet Over Medium-High Heat. Saute The Onions Until Browned And Caramelized; Set Aside. 3.Season The Chicken With Salt And Pepper. Place On Grill, And Cook Until No Longer Pink And Juices Run Clear, About 5 Minutes On Each Side. Place Buns On Grill Just Long Enough To Toast Them. 4.Spread Buns With Mayonnaise To Taste, Then Layer With Chicken, Caramelized Onion, Provolone And Avocado.