In a pot over medium heat, bring chicken, onion, garlic, chicken stock, and oregano to a simmer, then cover, reduce to medium low, and cook 20 minutes, or until internal temperature reaches 165 F. With tongs, transfer chicken and onion pieces to a plate and set aside to cool. Skim any white bits from the surface of the chicken stock, then bring to a low boil and reduce by half.
Meanwhile, purée cooled onions and chipotle peppers with a small food processor or immersion blender, adding some chicken stock if needed. Transfer mixture to stockpot, along with tomato paste. Shred chicken, adding it to the pot along the way. Continue cooking until desired thickness is reached.
In a large skillet over medium-high heat, warm tortillas, 10 to 20 seconds per side. Stack onto a kitchen towel and wrap up until ready to serve.
*To add more flavor without adding as much heat, use some of the sauce around the peppers instead of the peppers themselves.
Per serving (before toppings): 691 calories, 9 grams fat (1 gram saturated fat), 108 grams total carbohydrates, 86 milligrams cholesterol, 312 milligrams sodium, 6 grams fiber, 42 grams protein.
