In a large soup pot, over medium-high heat, add oil, onions, bell pepper, and garlic and sauté for 3 minutes.
Add chili powder and salt and allow spices to toast in the hot pan. Mix well with the sautéed onions and garlic for 2 minutes.
Using a fork, pull out two whole chipotles peppers in adobo sauce and add to the pot. Let the peppers cook for 1 minute, while stirring gently so as not to break open peppers. Add tomatoes and stock and bring to a boil.
Once the soup is boiling, carefully add the whole chicken breasts and poach in the soup for about 20 minutes, until cooked through.
In the meantime, prepare the corn tortillas, which will be the thickening agent for the soup. Place the torn corn tortillas in a large microwave-proof bowl with water, and heat on high for 2 minutes. Carefully remove the bowl of tortillas, which will now have absorbed most or all of the water and reserve to the side.
Preheat the oven to 350ºF, while the oven is preheating, take the remaining whole tortillas and place them on a cutting board. Stack the tortillas and with a sharp knife slice them through the center vertically and horizontally to form quarters.
Stack the quarters and slice into strips about an 1/8-inch wide. Place the strips on a baking sheet and toast for 10-15 minutes. Toss strips after seven minutes, to make sure that they toast evenly.
After 20 minutes of the chicken poaching, reduce the soup to a simmer. With tongs or two forks, carefully remove poached chicken from the soup, and set aside on a plate to cool slightly.
Add corn tortilla and water mixture into the soup base and stir to combine.
Once the chicken is cool enough to work with, use two forks to shred the chicken along the grain (pro-tip: look for the natural lines in the breasts and place both forks along the lines, pulling them apart in order to create nice long chunks of meat).
Return the shredded chicken to the soup to reheat while you prepare the garnish for your soup.
Pour the soup into bowls and garnish with avocado, crispy corn tortilla strips, cilantro, and lime.