In a 10" cast-iron skillet over high heat, toast chiles, turning once, until fragrant, about 2 minutes. Transfer to a small bowl and cover with 1 cup boiling water; let soak until soft, 20 minutes. Transfer chiles and their liquid to a blender along with 25 cloves garlic, orange juice, oil, honey, 1 tbsp. salt, and peppercorns and purée until smooth, about 2 minutes. Set chile rub aside.
Heat oven to 500° and place rack in bottom third of oven. Season inside of turkey with salt. Rub turkey with chile rub and stuff with onions and oranges; transfer turkey breast side up to a rack set in a roasting pan. Roast for 30 minutes; lower heat to 325° and cook for 30 minutes more. Remove turkey from oven and, using kitchen towels to protect your hands, flip turkey breast side down. Roast, basting occasionally, until an instant-read thermometer inserted into a thigh but not touching the bone registers 165°, about 3 hours total for a 12-lb. turkey. Lower oven to 150°. Transfer turkey, breast side up, to a baking sheet; return to oven to keep warm. Remove turkey from oven 15 minutes before serving.
Meanwhile, add port to roasting pan and heat over high heat. Scrape up any brown bits; cook, stirring, until mixture has reduced by half, about 5 minutes. Set a sieve over a bowl; strain liquid and discard solids. Put liquid into freezer and leave for 30 minutes. Skim and discard fat; set liquid aside. Melt butter in a 12" skillet over medium-high heat. Add remaining garlic; cook until soft, about 2 minutes. Add broth and reserved liquid; boil to reduce by half, about 15 minutes. Whisk in cornstarch mixture; boil; strain through a sieve into a 1-qt. saucepan. Stir in nutmeg and parsley; season gravy with salt and pepper. Carve turkey and serve with the gravy.