In 2 1/2-quart microwavable bowl, microwave cheese, salsa and barbecue sauce on High 6 to 9 minutes, stirring every 3 minutes, until cheese is melted. Stir in onions. Spoon cheese mixture into 1- to 1 1/2-quart slow cooker; set on Low heat setting to keep warm.
Heat chicken as directed on package.
To serve, have guests place chips on individual plates; spoon cheese mixture over chips and top with warm chicken, tomatoes, lettuce and sour cream.