Wash the oranges and lemon thoroughly. Place the whole fruit in a large pot, cover completely with water, bring to a boil, then reduce to a gentle simmer. Cook until a knife slides in easily, about 1 1/2 hours. Lift out the fruit and reserve the cooking liquid. Cool until easy to handle.

Cut the cooled oranges open. Collect all seeds and tie them in a small heat-safe bag or cloth. Separate the peel from the flesh. Slice the softened peel into neat strips and chop the pulp. Keep both together and weigh the prepared peel and pulp.

Return the peel and pulp to a clean pot and add an equal weight of granulated sugar. Add the seed bag. Pour in just enough reserved cooking liquid to help the sugar dissolve and the mixture move freely. Stir over medium heat until the sugar is fully dissolved. Avoid very high heat.

Increase the heat slightly and bring to a steady boil. Boil for about 15 minutes, stirring regularly. Test for set on a chilled plate. If it wrinkles when nudged after 30–60 seconds, it is ready. If not, boil a few minutes more and test again. Remove and squeeze the seed bag well, then discard.

Prepare a boiling water bath canner and sterilized jars. Ladle hot marmalade into hot jars, leaving 1/4 inch headspace. Remove air bubbles, wipe rims, and seal with lids and bands. Process in a boiling water bath for 10 minutes. Cool completely and check seals before storing.