How to Make It
In a very large skillet, heat the vegetable oil. Season the chicken with salt and pepper and add to the skillet, skin side down. Cook over high heat, turning once, until browned, about 8 minutes. Transfer the chicken to a plate and pour off all but 3 tablespoons of the fat. Add the mushrooms to the skillet and cook them over moderate heat, undisturbed, until they begin to brown on the bottom, about 2 minutes.
Meanwhile, in a food processor, puree the garlic cloves with the onion and cilantro leaves. Add the garlic and onion puree to the mushrooms and cook over moderate heat until very fragrant, about 1 minute. Stir in the tomato sauce. Return the chicken legs to the skillet and bring to a simmer. Cover and simmer the sauce until the chicken is cooked through, about 15 minutes. Season the braised chicken legs with salt and pepper, transfer them to plates and serve with the sauce.