Remove the buttons from the oranges and scrub them in warm water. Dry.
Cut around the middle of the oranges and juice. Add the lemon juice.
Collect the pith and pips of the oranges, and place into a muslin bag.
Using a teaspoon, carve out any loose stringy pith from each orange half and add to the muslin bag.
Fill your jam pan with 2.25 l of water
Strain juice of oranges and lemons into the jam pan.
Add any pulp and pips from the sieve into the muslin bag and tie tightly.
Add muslin bag to the pan of water and juice.
Cut orange peel into strips (cutting the hemispheres in half first makes this easier).
Strips can be anywhere from wafer thin to about 3mm wide, but best to be consistent with your size.
Add peel to the jam pan and cover, leaving overnight.
Bring the pan up to a bubble and leave simmering for 2 hours
The peel should now be soft and the liquid reduced by about 1/2
Remove the muslin bag, squeezing out any extra liquid.
Place your jars and lids into a preheated oven at 140 °C / 275 °F (120 °C / 240 °F fan).
Add the sugar and stir slowly, allowing it to dissolve.
Once it has fully dissolved, turn the heat up to maximum and bring the jam to a rapid rolling boil.
Boil for 5-10 minutes before testing for set.
Remove the pan from the heat, and place a teaspoon of the jam on a cold plate.
After a couple of minutes, push the jam with your finger. If it wrinkles, and doesn’t flow back into the gap, you have achieved a set.
If not, return the pan to the heat for 2 more minutes and try again.
When the surface of the jam is still, remove any scum from around the edges.
Then give the jam a thorough stir, to disperse any remaining scum and to speed up cooling.
Leave for 10 minutes, until the peel is suspended in the jam, rather than floating on the surface.
Remove your sterile jars from the oven, remembering not to touch the inside.
Use a jam funnel and ladle to fill the jars, and place the lids on, remembering not to touch the inside. A magnet can help to lift the jar lids from your baking sheet.
Secure the lids and leave to cool.