1.Wash and scrub oranges and lemon. Cut in half lengthwise then thinly slice. Place orange and lemon in a glass or ceramic bowl. Cover with 1 litre (4 cups) water. Cover bowl with plastic food wrap. Stand overnight.
2.Transfer orange mixture to a large heavy-based saucepan. Bring to the boil. Reduce heat to low; cover. Simmer for 1 hour or until zest is very soft. Add sugar; cook and stir for 5 minutes or until sugar has dissolved. Increase heat to high. Bring to the boil.
3.Boil, uncovered, for 15 minutes or until marmalade gels when tested or reaches 105°C on a sugar thermometer. Stand for 5 minutes before pouring into hot sterilised jars; seal while hot.
To test that jam gels: Place a teaspoon of marmalade on a cold saucer; place in freezer for 2 minutes. Run finger through marmalade. If surface wrinkles and jam separates, it’s ready.Note