Combine chicken & marinade ingredients in a ziplock bag, squish around to combine & coat thoroughly & refrigerate 2-4 hours, turning occasionally.
In a small bowl, stir together all mayonnaise ingredients & refrigerate 30 minutes.
Heat grill or broiler. Remove chicken from marinade; discard marinade. Gril or broil 12-14 minutes until browned & juices run clear, turning once. Serve with avocado slices & use mayonnaise as a dipping sauce.