Thinly slice the red onion and place in a small bowl with white vinegar and sea salt; let sit for 15 minutes to pickle.
In a small jar, whisk together tamari, apple cider vinegar, honey, and minced garlic to create the adobo sauce.
Heat avocado oil in a skillet over medium heat, add the chicken breast, and cook until browned on both sides.
Pour the adobo sauce over the chicken, cover, and simmer for 10-12 minutes until the chicken is cooked through and tender.
Remove the chicken from the pan, shred it with two forks, and toss it back into the remaining pan sauce.
Warm the corn tortillas in a dry pan or over an open flame until pliable.
Assemble the tacos by layering the shredded chicken, pickled onions, avocado slices, and fresh cilantro.