Trim the boneless chicken breasts if needed and place in a saucepan with 6 cups of the defatted chicken broth. Bring to a boil, then reduce heat. Simmer until the chicken tests done, about 15 to 20 minutes. Remove the chicken from the broth and shred the chicken breast. Reserve the broth for later.
While the chicken breasts are cooking, cut the corn tortillas into strips. Place the strips in a bowl and drizzle with 1 tablespoon of corn oil. Toss to coat, then spread evenly on a baking sheet. Bake in a preheated 400 degree oven for about 10 to 12 minutes until lightly browned and crisp, turning two to three times ensuring even baking. Remove to a serving bowl.
Add the 2 teaspoons corn oil to a small stock pot or Dutch oven set over medium heat. Add onions and the tomatillos and cook for about 5 to 8 minutes, until the onions have softened and are beginning to turn a light brown.
Stir in the chopped garlic cloves, and cook for another 2 minutes or until fragrant.
Add in the chopped tomatoes with liquid, green onions, 2 tablespoons chopped cilantro, cumin, paprika, and bay leaf. Strain the reserved chicken broth and pour over the tomato mixture.
Bring to a boil, then reduce heat and simmer for about 10 minutes.
Remove from heat. Stir in the Cotija cheese, and remove the bay leaf.
Serve the hot soup with the reserved shredded chicken breast and chopped avocado. Add a small dollop of sour cream on the top of each portion. Serve immediately.