Char the poblano chiles over a gas flame or in a broiler until blackened on all sides. Enclose chiles in a paper or plastic bag. Let stand 10 minutes to steam. Peel and seed chiles.
Toast ancho chiles in a heavy, large skillet over high heat until the color darkens slightly and the chilies are fragrant, about 30 seconds per side. Transfer the ancho chiles to a medium-size bowl.
Add enough hot water to the bowl to cover the chiles. Let stand until the chilies soften, about 20 minutes.
Purée four ancho chile halves with 1/2 cup soaking liquid in a blender. Add the roasted poblano chiles; Purée. Season with salt and pepper. Drain the remaining two chile halves; chill. (Purée and soaked chiles can be made one day ahead. Cover separately and chill.)
Preheat oven to 350° F.
Rinse the turkey inside and out, and pat dry. Sprinkle the turkey with salt and pepper. Cut the remaining two ancho chile halves into strips. Place the chile strips and onion in the turkey cavity.
Mix the butter and achiote paste in a small bowl to blend. Run your fingers between the turkey breast skin and meat to loosen. Rub half of the achiote butter over turkey breast under skin. Rub butter over the outside of turkey.
Place turkey in a large roasting pan. Tuck wings under the turkey. Tie legs together to hold shape. Pour 1 1/2 cups of stock into the pan.
Roast the turkey 45 minutes. Tent the turkey loosely with foil. Continue roasting until a meat thermometer inserted into the thickest part of the thigh registers 180° F, basting every 30 minutes with pan juices, about 3 1/2 hours. Transfer the turkey to platter. Tent with foil.
Pour the turkey pan juices into a measuring cup. Spoon off the fat from the pan juices, reserving 1/4 cup of fat. Add enough remaining stock to the pan juices to measure 3 cups. Return 1/4 cup fat to the roasting pan.
Place the pan over two burners set at medium heat. Add the Masa Harina; whisk until the mixture resembles a paste, scraping up any browned bits, about 2 minutes. Gradually whisk in the pan juices.
Add chile Purée; simmer 4 minutes to blend the flavors. Season gravy with salt and pepper. Serve turkey with gravy.Servings: 14