To prepare soup, combine the onions and tomatoes in a blender or food processor, and process until smooth. Heat oil in a Dutch oven over medium-high heat until hot. Add the dried oregano, ground cumin, and minced garlic, and sauté 10 seconds. Add pureed onion mixture to pan (mixture will boil vigorously), and cook 1 minute. Stir in Chicken Stock and salt; bring the mixture to a boil. Reduce heat, and simmer 15 minutes.
Preheat oven to 350°.
To prepare toppings, place the tortilla strips in a single layer on a jelly-roll pan. Bake at 350° for 12 minutes or until tortilla strips are toasted.
Ladle soup into each of 6 bowls, and top with the baked tortilla strips, shredded chicken, and cheese. Serve with lime wedges.