Heat the olive oil in a large pot over medium heat. Once the oil is glistening, add half of the onions and cook until beginning to soften, about 5 minutes. Add the garlic and cook, stirring, until fragrant, about 1 more minute.
Add the chicken, chicken stock, orange juice, bay leaf, and taco seasoning. Increase the heat to high and bring to a boil, then reduce to a simmer. Cover and cook for about 45 minutes, or until the chicken is very tender and can be pulled apart easily with a fork.
Turn on the broiler.
Using a slotted spoon, transfer the chicken to a rimmed sheet pan, reserving the liquid in the pot. Use two forks to shred the chicken into 1-inch pieces and spread into an even layer. Brush the chicken with about ¼ cup of the reserved liquid. Broil, tossing halfway through, until the chicken is crispy, about 10 minutes.
Meanwhile, in a small bowl, combine the remaining onion with cilantro.
Serve the chicken, tortillas, and onion mixture family-style.
