How to Make It
In a small skillet, toast the chiles until they just start to blister, about 4 minutes. Transfer the chiles to a small bowl and cover with boiling water. Let stand for 15 minutes. Drain and pat dry.
In a food processor, combine the chiles, butter, garlic and the chopped scallions and puree until smooth. Season generously with salt and pepper. Carefully loosen the turkey breast skin and rub one-third of the ancho-scallion butter over the meat. Rub the remaining butter all over the top skin and bottom of the turkey breast. Season with salt and pepper. Let the turkey breast stand at room temperature for 1 hour.
Preheat the oven to 400°. Set the turkey breast skin side down on a work surface and fold the sides of the breast into the center. Tie the turkey in 5 places with kitchen string to make a neat roast. Set the turkey skin side up, in a shallow roasting pan.
Roast the turkey breast for 20 minutes, until the top just starts to brown. Reduce the oven temperature to 350° and roast for 35 to 40 minutes longer, until an instant-read thermometer inserted in the thickest part of the meat registers 145°. Transfer the turkey to a carving board and let stand for at least 10 minutes.
Meanwhile, spoon off all but 1 tablespoon of fat from the roasting pan. Add the sliced scallions and the flour and cook over moderate heat for 3 minutes. Add the turkey stock and boil over moderately high heat until lightly thickened, about 5 minutes. Season with salt and pepper. Discard the strings and carve the turkey in 1/2-inch-thick slices. Serve with the gravy.