Heat the olive oil and butter in a large Dutch oven over high heat. Add the chicken a few pieces at a time and brown on all sides. As they brown, sprinkle with salt and pepper and transfer to a platter.
Add the onions to the pot and cook over medium heat to soften. Add the celery and garlic and cook until they turn a rich brown, about 5 minutes. Add the tomatoes and cook, stirring, for 3 more minutes. Deglaze with vermouth, then add the chicken broth and thyme. Return the chicken to the pot and bring to a simmer. Lower the heat, then cover and simmer for 45 minutes. Add the roasted peppers and pasilla, then cover and cook until the chicken is fork-tender and the thigh bones are loose, an additional 30 minutes.
This recipe has been updated and may differ from what was originally published or broadcast.