Place chicken breast halves in a bowl with the lemon juice, olive oil, and lemon pepper. Stir to coat, and marinate in the fridge for at least 1 hour.
Preheat a grill for high heat.
Lightly oil the gBBQ plate. Place the onions and mushrooms on the BBQ, and cook until lightly charred on both sides; set aside. Place chicken onto the BBQ, and discard the marinade. Cook for 15 minutes, turning once, or until juices run clear. Remove from heat, cool, and chop.
In a large bowl, thoroughly mix the onions, mushrooms, chicken, and mayonnaise. Season with hot sauce, salt, and pepper. Cover, and refrigerate until serving.