- On a clean surface, slice the peels and pith off the oranges and clementines. Cut the flesh into 2.5-cm (1-inch) cubes. Set aside. Finely chop the peels after scraping off any pith.
- Squeeze the juice from the lemon. Remove the white membranes. Finely chop the peel.
- Place the citrus peels in a large saucepan, add cold water to cover and bring to a boil. Drain and repeat twice.
- In the same saucepan, combine the citrus peels, citrus flesh, orange juice, sugar and cranberries. Bring to a boil, reduce the heat to medium and simmer for 1 hour, stirring frequently, until the mixture is translucent. To check whether the marmalade is done, do a plate test (see note) or set a candy thermometer in the centre of the saucepan and simmer until it reads 104°C (219°F). During cooking, carefully skim off any foam.
- Add the lemon juice and cook for 1 more minute.
- Pour into sterilized jars.