Lay the chicken thighs on a sheet pan and season both sides with 2 teaspoons of the salt.
Turn a gas burner to medium. Place the poblano peppers on the flame and rotate frequently until they are charred all over. WARNING: this will result in bits of the skin wafting up as ash. Be careful and turn your exhaust hoods on. (Alternatively, turn the broiler on high. Place peppers on a sheet pan and broil, turning occasionally until charred. WARNING: Never walk away from something under the broiler as it can burn quickly.)
Place the charred poblanos in a glass bowl and cover tightly with plastic wrap. Allow to steam for 10-15 minutes.
While the poblanos steam, finely chop the onions, jalapeños, and serrano if using.
Remove the poblanos from the bowl and place on a cutting board. Cut them open, cut off the tops and remove the ribs and any remaining seeds. Flip the peppers interior side down and use a knife blade to scrape off the charred outer skin. It may not all come off and that's okay.
Cut the poblanos into wide strips, then into ½ inch squares.
Add 2 tablespoons of the olive oil to a 6-quart Dutch oven over medium heat. Once the oil shimmers, add the onions and chopped peppers and ½ teaspoon of salt. Sweat them until tender and onions are translucent, about 6 - 8 minutes.
Push the peppers and onions aside, add one more tablespoon of olive oil, add the cumin and coriander to make a paste with the olive oil and let it sizzle for a minute. Stir the spices into the mixture for about 1 minute.
Use a microplane grater to grate the garlic cloves, add to the pan, and stir into the onion mixture until fragrant, about 1 minute.
Add the chicken stock, beans including the liquid in the cans, 1 teaspoon salt, and chicken thighs to the pot and stir. Bring to a boil, then reduce to a simmer. Simmer for about 90 minutes, skimming the fat and stirring occasionally, until it has thickened and reduced by about ¼.
Remove the chicken thighs to a plate or small dish. Use two forks to pull the chicken into shreds and add back into the pan.
Stir in the chicken, then taste for seasoning. If it needs salt, add some and simmer for about 5 more minutes. Taste and add more salt if necessary. Remove from heat, transfer to bowls and serve.