Slow Cooker Method 

1. Heat the olive oil in a large skillet 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. 
2. Transfer the onion mixture to the slow cooker. Add the chicken, chicken stock, orange juice, bay leaf, taco seasoning and stir to combine. Cover and cook on High for 4 hours or Low for 8 hours.
3. Transfer the chicken to a rimmed sheet pan. 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. 
4. Turn on the broiler and broil, tossing halfway through, until the chicken is crispy, about 10 minutes. 
5. Meanwhile, in a small bowl, combine the remaining onion with cilantro. 
6. Serve the chicken, tortillas, and onion mixture family-style. 

Instant Pot Method

1. Add the olive oil to the Instant Pot, hit Sauté, and adjust to select the More or High setting. After 3 minutes of the oil heating, 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. 
2. Add the chicken, chicken stock, orange juice, bay leaf, taco seasoning and stir to combine. 
3. Secure the lid, moving the valve to the sealed position. Hit Cancel and then hit Manual or Pressure Cook. Pressure cook for 8 minutes. Let the pressure release naturally for 10 minutes, then manually vent to release the remaining pressure.
4. Transfer the chicken to a rimmed sheet pan. 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 remaining liquid. 
5. Turn on the broiler and broil, tossing halfway through, until the chicken is crispy, about 10 minutes. 
6. Meanwhile, in a small bowl, combine the remaining onion with cilantro. 
7. Serve the chicken, tortillas, and onion mixture family-style.