Put the chicken thighs in a pot with the broth, the whole half onion, 2 garlic cloves, the bay leaves and 1 teaspoon of the salt. Bring to a bare simmer and poach 18 to 20 minutes, until cooked through.
Lift the chicken out and let it cool enough to handle. Keep the poaching liquid; strain it and set 1 cup aside.
While it cools, simmer the tomatoes in water for 8 minutes, until the skins split and loosen, then drain and slip the skins off.
Blend the peeled tomatoes with the chipotles, their adobo sauce, the remaining 2 garlic cloves and the oregano until smooth.
Shred the chicken with two forks, along the grain first and then across, into pieces rather than threads.
Heat the oil in a wide pan over medium-high and cook the sliced onion 5 minutes, until soft at the edges.
Pour in the blended sauce all at once and let it fry hard for 6 to 8 minutes, stirring, until it darkens, thickens and pulls away from the side of the pan.
Add the shredded chicken, the reserved cup of poaching liquid and the remaining salt. Simmer 8 to 10 minutes, until the sauce clings to the meat and no loose liquid pools in the pan.