Put the whole oranges in a pan, cover with water and boil, covered, for 1 hr until a knife easily pierces them. If the oranges won’t stay under the water, place a small saucepan lid directly on top to keep them submerged. Remove the oranges from the water and cool, then quarter and remove any seeds and obvious pith where the stalk was. Blitz the oranges to a rough purée with a hand blender or in a food processor and put in a large bowl.