Chicken Tortilla Soup

In a 3-quart saucepan simmer chicken in water until tender*, about 25 minutes. Remove chicken from stock, straining and reserving stock; allow chicken to cool enough to handle. Separate meat from skin and bone, and dice. Set aside meat and stock. In a large stockpot over medium-high heat, heat 2 tablespoons of the oil and cook onions and garlic until onion is tender, about 3 minutes. Stir in chicken meat, stock and broth, tomato sauce, green bell pepper, red pepper flakes, basil, salt and pepper. Heat to boiling. Reduce heat and simmer, uncovered, for 30 minutes. In a skillet, heat remaining 8 tablespoons (1/2 cup) oil. Add tortilla slices, working in batches if necessary, and cook until light brown, 30 to 60 seconds; drain on paper towel. Divide tortilla strips among the serving bowls; pour soup on top. Top with cheese and avocado.