Saute the onion and garlic in the vegetable oil in a large stockpot. Add the beans, tomatoes, and broth, stir until smooth and bring to a boil. Reduce heat and simmer 15 minutes. Stir in cilantro. Cut tortillas in thin trips, spread in a single layer on a baking sheet. Bake 15 minutes at 350 degrees until browned, stirring every 5 minutes. Cool. Ladle soup into bowls and top with tortilla strips, cheese, and sour cream. Serve immediately.