Halve the oranges and squeeze the juice. Quarter, then scrape away the membrane and pith, leaving a thin layer of pith still attached to the rind. Tie the membrane, pith and pips in a square of muslin. Thinly slice the orange rind.
Remove the trivet from the base of the pressure cooker. Add the shredded orange rind, orange juice, lemon juice and muslin bag to the pressure cooker, then pour on the measurement water. Cover with the pressure cooker lid, bring up to medium pressure, then reduce the heat slightly so that the pressure cooker gently hisses and indicates that the correct pressure has been reached. Cook for 10 minutes, then leave to cool and allow the pressure to release slowly.
Remove the pressure cooker lid. Lift the muslin bag a little above the contents of the pressure cooker and press between two wooden spoons to remove as much pectin-rich juice as possible, then discard the bag. Check that the rind is just tender – if not, bring back to pressure and cook for a further 2–3 minutes.
Pour in the sugar and heat gently, without the lid, until the sugar has dissolved, stirring from time to time. Bring to the boil, then boil rapidly until setting point is reached (10–20 minutes). Skim with a draining spoon or stir in butter if needed. Allow to stand for 10 minutes.
Ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.