1. Soak Ancho chilli in boiling water for 15 minutes, just to reconstitute them, and then chop them into slices. Set it aside.
2. In food processor, combine ancho chilli, tomatoes, garlic, and red onion. Blend.
3. Take a skillet and heat olive oil in it. Pour tomato mixture and cook for few minutes for the flavor to infuse.
4. In the meantime, line a baking sheet with foil, place tortilla strips on it, drizzle with olive oil, and toss well.
5. Season with salt and pepper.
6. Pop it in oven and broil for about 5 minutes or until crisp.
7. In the skillet, add chicken stock, rice and shredded chicken pieces. Mix and allow it to simmer. Cook until done.
8. Transfer soup to serving bowl, top it with tortilla, cheese, and avocado.
9. Squeeze lime juice, and serve tortilla soup hot.
You can use ancho chilli with seeds if you want your soup to be spicy.