Preheat oven to 200 degrees C. Salt and pepper the duck both inside and out. Stuff the duck with 1 sprig of thyme, garlic and half the orange. Rub the outside with the other sprig of thyme.
Place duck on a rack in a large roasting tin. Roast in the middle of the oven for 1 hour per kg (so 1 1/2 hours for a 1 1/2 kg duck). Baste at least twice with the fat the duck gives up. 1/2 hour before the end of the cooking time, place orange slices over the duck and cover with a sheet of aluminium foil, shiny side down.
Place the duck on a cutting board to rest and make a gravy with the drippings: add a little water, salt and pepper and whatever remains of the second sprig of thyme. Boil 1 minute and place in a sauce boat.