Heat oven to 200°C/180°C Fan/Gas Mark 6. Remove all the fat from inside the bird and, using a small knife, prick the skin all over including under the wings and around the legs. Sit the goose in an empty sink and slowly pour over 2-4 kettlefuls of boiling water. The skin will tighten and become shiny. Leave to cool, then pat dry.
Stuff the goose with onion, orange and bay. Rub all over with the oil and season with salt. Sit the goose on a rack in a large roasting tin and tightly cover with a large piece of foil. Cook for 1 hr 30 mins.
Remove from the oven, remove the foil and use oven gloves to lift the rack (and the goose) out of the roasting tin. Pour all the fat from the tin into a bowl and set aside. Sit the goose back in the tin, cover with foil, then roast for 1 hr. Remove the foil and return the bird to the oven for 15-20 mins to brown the skin.
Transfer to a large board or platter to rest, uncovered, in a warm place for 30 mins.