Heat oil in a large skillet over medium-high heat and brown the chicken breasts on both sides.
Remove chicken, add onions to the skillet, and sauté for 5 minutes until soft and translucent.⏱ 5 min
Stir in garlic, chili powder, and cumin, cooking for 1 minute until fragrant.⏱ 1 min
Add the tomato sauce and 1/4 cup water, then return the chicken to the pan.
Cover and simmer on low heat for 15 minutes until the chicken is cooked through.⏱ 15 min
Transfer the chicken to a board, shred it using two forks, and return it to the sauce.
Simmer uncovered for 5 minutes until the sauce coats the chicken nicely.⏱ 5 min
Warm the tortillas in a dry pan and fill with the chicken mixture.