Cooking the onions, ancho chile, aromatic thyme and chicken together is an effortless way to prepare a sauce as part of a one-pot meal. The mild chile sauce could also be pureed with an immersion blender. Slideshow: More Chicken Thigh Recipes
Preheat oven to 325°. In a small skillet set over medium heat, toast the chile until it changes color slightly and turns soft, about 2 minutes. Remove the chile and when cool enough to handle, remove the seeds and stem and tear the chile into large pieces.
Season the chicken thighs with salt and pepper. In a large skillet or Dutch oven set over medium-high heat, add the oil and heat until it shimmers. Brown the chicken thighs skin side down, about 5 minutes. Turn over and brown all other sides, about another 5 minutes.
Drain off most of the chicken fat, then add the onion. Cook, stirring, until onion is fairly soft and translucent, about 10 minutes. Add the thyme leaves and cook 1 minute. Add the broth, torn chiles and thighs, skin-side up. Cover and place in the oven. Cook for 15 minutes, then flip the thighs over. Cook until the thighs are tender and falling off the bone, about another 15 minutes
Remove the thighs and skim off fat from sauce. Place sauce in a blender and puree until smooth. Add the vinegar. Serve the thighs with the sauce in bowls.
The chicken can be prepared through step 3. Separate the sauce and thighs and refrigerate. When preparing day of, skim the fat from the sauce, then warm the chicken in the sauce and proceed.