Lightly coat the inside of your slow cooker with cooking spray. This helps prevent sticking and makes cleanup much easier later. Pour half of the diced tomatoes with green chiles into the bottom of the cooker.
Place the chicken breasts directly on top of the tomato mixture. Spread them out evenly so they cook consistently. Trim any excess fat before adding them to the slow cooker.
Pour the remaining tomatoes over the chicken. Add the diced onions evenly across the top. This layering helps distribute flavor throughout the cooking process.
In a small bowl, combine the oregano, cumin, garlic powder, cayenne pepper, black pepper, and salt. Sprinkle the seasoning mixture evenly over all the ingredients.
Cover the slow cooker with the lid and cook on Low for 6–8 hours or High for 3–4 hours. The chicken should become fork-tender and easy to shred when fully cooked.
Before shredding, carefully remove about one cup of the cooking liquid. This prevents the taco filling from becoming too wet and helps maintain the perfect texture.
Use two forks to pull the chicken apart into bite-sized shreds. You can do this directly in the slow cooker or transfer the chicken to a cutting board first.
Squeeze the fresh lime juice over the shredded chicken. Stir everything together so the chicken absorbs the remaining flavorful juices and citrus brightness.
Heat your tortillas before serving for the best texture and flavor. Fill each tortilla with the shredded chicken and your favorite toppings.