Cut tortillas in half then slice each into ¼-inch wide strips. Heat oil in a stockpot and fry till lightly browned and crisp. Drain on a paper towel-lined plate.
Add chopped onions to the pan and cook for a couple of minutes. Add chiles and garlic and cook just a minute more.
Add stock, tomatoes, corn, cumin, oregano, and salt to taste.
Bring to a boil and then lower to a simmer. Cover and cook for 15 minutes, then add chicken and cook till heated through.
Serve with avocado, cheese, cilantro, tortilla strips, and a sprinkle of lime juice if desired.
