In a large, deep pot or Dutch oven, heat the oil over medium heat and sauté the onions until soft. Remove the chicken from the marinade and add to the pot, skin side down. Cook to render the fat and begin browning the skin, about 5 minutes, then flip and sear the other side.
Pour the marinade and water into the pot, stir and set the pot over high heat, bringing to a boil. Reduce the heat, cover and simmer, stirring occasionally, until the chicken is cooked through and very tender, about 30 minutes. Remove the lid and continue cooking until the sauce is reduced and becomes thick and creamy, about 10 to 15 more minutes. (If you prefer an even thicker sauce, remove the chicken and cook sauce over high heat and simmer until further reduced.) Serve the chicken and the sauce over rice or a whole grain.
To create a crisped skin on the chicken: Remove the chicken from the sauce and place in a roasting pan. Preheat the broiler to high and run the chicken under the broiler until the skin is browned and crisp, about 3 to 5 minutes.