Place chicken breasts, chicken broth, onion, chili powder, cumin, salt, and pepper in a large pot or slow cooker.
Stovetop Method: Bring to a boil, then reduce heat to low, cover, and simmer for 20-25 minutes, or until chicken is cooked through.
Slow Cooker Method: Cook on low for 4-6 hours or on high for 2-3 hours.
Remove the chicken from the liquid and place it on a cutting board. Shred the chicken using two forks.
Return the shredded chicken to the pot and stir it into the remaining cooking liquid to coat it well.
Warm the tortillas according to package directions (in a dry skillet or microwave).
Fill each tortilla with the shredded chicken mixture. Add your desired toppings.
Notes
For extra flavor, add 1 tablespoon of lime juice to the chicken mixture after shredding.
If you prefer a thicker sauce, remove about 1/2 cup of the cooking liquid and simmer it separately until reduced by half, then stir it back into the shredded chicken.