Sauté onion, jalapeño pepper, garlic, and tomatoes in a large kettle for several minutes. Add remaining ingredients except tortillas, avocado, cheese, and sour cream. Simmer for 1 hour. About 10 to 20 minutes before serving, tear tortillas into bite size pieces and add to soup. Place cubed avocado and grated cheese in individual bowls and ladle hot soup on top. Top with spoonful of sour cream (makes 4-6 servings)