Heat the vegetable oil in a 4 qt (4lt) Dutch oven over medium heat. Add the onions and allow to brown until translucent. Chop the chicken breasts into several pieces, keeping the rib bones attached. Add the chicken breasts to the Dutch oven, and allow to cook until well browned, about 15 minutes. The chicken does not need to be completely cooked.

Add the chicken stock and allow the Dutch oven to return to a low simmer. Add the garlic, cumin, salt and pepper. Cover the pot, and simmer for about 20 minutes.

Meanwhile, prepare the chiles. Roast the uncooked chiles over a gas flame or under the broiler in an electric oven. Roast until well browned and blistered. Remove the chiles from the heat and place in a clean kitchen towel. Roll the chiles and towel into a bundle and then place the bundle in a paper bag so that the chiles sweat for 10-20 minutes.

Remove the chiles from the bag and bundle, peel away the loosened skin, and remove the stem and seeds. Rinse any remaining seeds away under running water or in a bowl of water. Cut the chiles into strips and then dice.

Remove the pieces of chicken breast from the pot wit a fork or slotted spoon. Remove and discard the rib bones. Using a cleaver, chop the chicken meat finely. Return the chicken meat to the pot and add the dry corn tortilla mix. Add the prepared chiles to the Dutch oven. Simmer for another 5 minutes.

Serve the white chili over rice and garnish with shredded cheddar cheese, chopped green onions and sliced avocado.