Place the chicken breasts in the slow cooker. Sprinkle the diced onion over the top.
Add chicken broth, cumin, chili powder, garlic powder, oregano, salt, and pepper. Add in the Rotel with the juices. Stir gently to combine.
Cover and cook on LOW for 6 hours or HIGH for 4 hours, until the chicken is tender and shreds easily.
Remove the chicken from the slow cooker, shred it using two forks, and return it to the slow cooker.
In a small bowl, whisk together the heavy cream and cornstarch until smooth. Slowly stir this mixture into the slow cooker.
Add the torn corn tortillas and shredded cheddar cheese. Stir everything well and cover the slow cooker again. Cook on HIGH for about 20-30 minutes, or until the cheese is melted and the mixture is bubbly.