1. 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.
2. 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.
3. Prepare grill.
4. 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.
5. Lightly coat tortillas on each side with cooking spray. Place tortillas on grill rack; grill 2 minutes on each side or until lightly browned.
6. 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 jalapeños, sour cream, and cilantro, if desired.
Achiote paste is made in part from ground annatto seed. It is a deep red, mildly spicy seasoning; you can find it at large supermarkets or Latin grocery stores.
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