Add 2 tablespoons of canola oil to a large skillet over medium heat. Allow the oil to get hot enough that a sprinkle of water sizzles when it hits the surface. This will create a great sear.
Carefully place the 3 boneless and skinless chicken breasts in the skillet. Sear them until they are lightly golden brown on both sides, approximately three to four minutes per side. This step is crucial for developing flavor.
Once the chicken is seared, take it out of the skillet and set it aside on a plate. In the same skillet, add the 6 cups of chicken stock, followed by the diced yellow onion, minced garlic, diced bell peppers, and all the spices: salt, black pepper, cayenne, cumin, and chili powder. Stir gently to combine.
Transfer this mixture to the slow cooker. Add the seared chicken breasts back into the pot along with 1 tablespoon of tomato paste. Give it a good stir to ensure everything is well mixed.
Cover the slow cooker and set it to cook on low for five to six hours or on high for three to four hours. The goal is for the chicken to be fully cooked and tender.
When the cooking time is nearly up, use a meat thermometer to check the internal temperature of the chicken. It should read at least 165 degrees Fahrenheit to ensure it's safe to eat.
About 15 minutes before serving, add the reserved juice of the lime and the thawed frozen corn kernels to the slow cooker. Cover and cook for an additional three to five minutes.
Once done, carefully remove the chicken from the slow cooker. Using two forks, shred it into bite-sized pieces. This will create a lovely texture for the soup.
To serve, ladle the Slow Cooker Chicken Fajita Tortilla Soup into bowls and top it with crispy tortilla strips, diced cilantro, and slices of avocado, if desired.
To make the crispy tortilla strips, preheat your oven to 350 degrees Fahrenheit. Cut tortillas into strips and lay them on a nonstick baking sheet. Lightly spray both sides with cooking spray and sprinkle with salt and pepper. Bake for about 20 minutes or until golden and crispy.