Combine all ingredients for the marinade in a bowl except the chicken and stir together with a fork. Add the chicken to the marinade and flip a few times to fully coat.
Marinade the chicken for 30-60 minutes and preheat the oven to 450F.
Remove chicken and onions from marinade and put on a greased rimmed baking sheet with the onions on top of the chicken breasts. Pour remaining marinade over the chicken and onions. Roast at 450F for 20 - 35 minutes or until chicken is 165F at its thickest part. Time will vary depending on how thick your chicken breasts are. To speed up cooking, take the marinating chicken out of the refrigerator 10 minutes before cooking.
Remove the chicken from the oven and remove to a cutting board with the onions. Slice the breasts down the middle long ways, and then do 3 or 4 cross cuts on each breast. This makes it easier to shred. Use 2 forks and shred the chicken.
Serve as part of a pita sandwich with hummus as a sauce or over yellow rice. Use vegetables like diced onion, cucumber, or cilantro to garnish.