Boil the tomatillos. Peel and quarter one of the onions. Place in a large saucepan with the tomatillos, garlic, and jalapeños. Fill with water and place over medium-high heat. Bring to a boil, then reduce to a simmer and cook until the skin on the tomatiilos just begins to crack, about 5 minutes.

Blend sauce. Drain and transfer to a blender. Add 1/2 of the cilantro (yes, the stems and everything) and a teaspoon of salt. Blend until smooth. Be careful blending hot ingredients. Start the blender off low and gradually increase the speed so it doesn't explode on you.

Brown the chicken. Heat oil in a large Dutch oven or soup pot over medium-high heat. Add chicken and season generously with salt and pepper. Cook, stirring occasionally, until starting to brown. Remove to a plate with a slotted spoon leaving the oil in the pot.

Saute the vegetables. Return the pot over medium-high heat and add the chopped onion, carrot, and celery and season generously with salt and pepper. Cook until the vegetables soften and begin to get some color, about 8-10 minutes.

Add remaining ingredients. Add chicken back into the pot along with the tomatillo sauce, butternut squash, cumin, and chicken broth. Bring to a boil then reduce to a simmer and cover. Let cook until the butternut squash and chicken are cooked through, about 25 minutes.

Serve. Taste and add a teaspoon or so more of salt. Chop the remaining 1/2 bunch of cilantro and stir into the soup. Serve with thinly sliced scallions on top.