Combine yogurt with spices, sugar, and lemon juice. Set aside and let marinate for 1 and a half hours.
Char the chiles over an open flame. You want them blackened all over; a wood fire lends a little smoke aroma but you can also just set them over a gas burner or blast them with a blow torch, turning them a few times until charred all over, 4 to 5 minutes total.
Transfer to a metal bowl and cover tightly with plastic wrap. Let the chiles steam for 10 minutes then remove them from the bowl and slip off the charred skins (gloves help here so you don’t end up with chile-hot fingertips!).
Cut the chiles—ribs, seeds, and all—into 1/2-inch dice and set aside; you should have about 2 1/2 cups. For this recipe, we keep in the chile ribs and seeds for extra spiciness.
Heat your slow cooker to high. Add 2 tablespoons of the oil and swirl to coat the bottom.
Add the onion, carrots, celery, garlic and 2 teaspoons of the salt and cook until the onions are soft and translucent, about 10 minutes, shaking the pan now and then.
Add the remaining 1 tablespoon of oil and stir in the potatoes; cook until the potatoes are fork tender, about 10 minutes more.
Grind the coriander and cumin seeds to a fine powder using a spice grinder or mortar and pestle.
Stir the coriander/cumin blend and the oregano into the vegetables.
Cook for 5 minutes to heat up and toast the spices and herbs, which helps the flavors release into the stew.
Add the tomatoes with their juice and the chicken stock and bring the mixture to a boil.
Add the chicken meat and chiles and turn the slow cooker to low. Cover tightly and allow the mixture to cook for 2 hours. At 2 hours, turn the slow cooker to the lowest setting and keep it there until ready to serve.
Just before serving, add the juice of one lime and the chopped cilantro. Adjust the seasoning if necessary with more salt.