To prepare pickled onions, combine onion and 2 cups boiling water in a large bowl. Cover and let stand 15 minutes; drain.
Add 1/2 cup orange juice, 1/2 cup vinegar, and 1/8 teaspoon salt to onions. Cover and chill at least 2 hours.
To prepare chicken, heat a small nonstick skillet over medium heat.
Add garlic to pan; cook 7 minutes or until tender, stirring occasionally.
Remove garlic from pan; peel and mash.
Combine garlic, 1/2 cup orange juice, and the next 7 ingredients (through pepper) in a large zip-top plastic bag. Seal bag, and shake vigorously to dissolve achiote paste.
Place each chicken breast half between 2 sheets of heavy-duty plastic wrap, and pound to 1/2-inch thickness using a meat mallet or small heavy skillet.
Add chicken to bag, and seal and marinate in refrigerator for 30 minutes, turning bag occasionally.
Prepare grill.
Remove chicken from bag; discard marinade.
Place chicken on grill rack coated with cooking spray; grill 5 minutes on each side or until done.
Let stand 15 minutes. Shred with 2 forks.
Lightly coat tortillas on each side with cooking spray.
Place tortillas on grill rack; grill 2 minutes on each side or until lightly browned.
Arrange 2 tortillas, overlapping halfway, on each of 6 plates. Top each serving with about 1/2 cup chicken mixture and about 1/2 cup pickled onions.
Drizzle evenly with vinegar mixture. Top each serving with 1/3 cup lettuce, 2 tablespoons avocado, and 2 tablespoons cheese.
Garnish with jalapeos, sour cream, and cilantro, if desired.