Step 1

Heat 3 tablespoons of the olive oil in a large nonstick skillet. Season the chicken all over with salt and pepper, add it to the skillet and cook over moderately high heat, turning once, until browned, about 12 minutes. Transfer the chicken to a 9-by-13-inch baking dish and pour off the fat in the skillet.

Step 2

Add the remaining 3 tablespoons of olive oil to the skillet along with the onion. Cook over moderately high heat, stirring occasionally, until the onion is lightly browned and softened, 5 minutes. Add the garlic and cook until fragrant, 2 minutes. Add the tomatoes and their juices, the chipotles and the broth and bring to a boil. Simmer over moderate heat, stirring occasionally, until thickened and slightly reduced, 20 minutes.

Step 3

Preheat the oven to 350°. Transfer the sauce to a food processor and let cool for 15 minutes. Puree until smooth and season with salt and pepper. Pour the sauce over the chicken. Bake the chicken uncovered in the center of the oven for about 45 minutes, until the meat is tender and the sauce is very thick and darkened around the edges. Wrap the tortillas in foil and warm them in the oven for about 10 minutes.

Step 4

Remove the chicken from the sauce and shred the meat; discard the bones. Return the chicken meat to the sauce. Spoon about 3 tablespoons of chicken onto each tortilla and sprinkle with the crumbled cheese. Garnish the chicken tacos with the scallions and chopped cilantro and serve hot.