How to Make It
In a spice grinder, grind the oregano, cumin, peppercorns and cloves to a powder. On a work surface, using the side of a heavy knife, mash the garlic with 2 teaspoons of salt and transfer to a small bowl. Stir in the 1 1/2 tablespoons of vinegar and the ground spices.
Rub 1 tablespoon of the spice paste onto the meaty side of each turkey breast half and 1/2 tablespoon of the paste onto the skin side. Let stand at room temperature for 1 hour.
Meanwhile, put the onions in a large bowl and add the 1 cup of vinegar. Toss well and let stand for 1 hour.
In a large enameled cast-iron casserole, bring the water to a boil. Stir in the remaining spice paste and add the turkey breast halves, skin side down. Cover partially and simmer over moderately low heat for 30 minutes. Turn the breasts over and simmer until just cooked through and tender, about 30 minutes longer.
Meanwhile, roast the poblanos directly over an open flame or under a preheated broiler, turning, until blackened all over. Transfer the poblanos to a small bowl, cover with plastic wrap and let stand for 5 minutes. Discard the blackened skin, the stems and seeds and cut the poblanos into thin strips.
Transfer the turkey breasts, skin side up, to a baking sheet and let cool until warm. Cover with plastic so the meat doesn't dry out.
Drain the onions and add to the cooking liquid. Cover and simmer over moderate heat, stirring occasionally, until tender, about 12 minutes. Uncover and simmer over moderately high heat for 3 minutes. Add the poblano strips and simmer until heated through, about 3 minutes. Season the cooking liquid with salt.
Preheat the broiler. Brush the skin with oil and broil the turkey breasts about 6 inches from the heat for about 2 minutes, until browned and crisp, rotating the pan as needed. Transfer the turkey to a carving board and let rest for a few minutes. Slice the turkey about 1/3 inch thick and arrange the slices on a platter. Spoon the onion-and-poblano cooking liquid over the turkey breasts and serve with Chile-Herb White Rice.