In a large bowl, whisk all the ingredients for the marinade and set aside.
Pound the breasts to even thickness or slice in half horizontally to get evenly sized breasts and add to the bowl of marinade. Mix through until the chicken is fully coated in the marinade. Use right away if needed or marinate for 30-minutes or up to 12 hours.
Grill chicken over medium-high heat or in a large heavy-duty skillet on the stovetop for 5-6 minutes per side or until the inside is cooked through and the outside is charred. Top with fresh avocado salsa and serve immediately.
To Make Avocado Salsa
While the chicken is grilled, combine all the ingredients for the salsa in a small bowl. Cover with plastic wrap in the fridge until ready to use.