Peel the oranges and remove as much of the white pith as possible to avoid bitterness.
Cut the oranges into small pieces, discarding any seeds.
Cook the Jam:
Place the orange pieces in a large saucepan over medium heat.
Add the sugar to the pan and stir to combine.
Bring the mixture to a boil, then reduce the heat to low. Let it simmer, stirring occasionally, for 45-60 minutes until the mixture thickens.
If using, stir in the ground cinnamon during the last 10 minutes of cooking.
Test the Jam:
To check if the jam is ready, place a small amount on a chilled plate. Let it cool for a few seconds, then push it with your finger. If the jam wrinkles and holds its shape, it is done. If not, continue simmering and test again after a few minutes.
Jar the Jam:
Once the jam has reached the desired consistency, remove it from heat.
Carefully spoon the hot jam into sterilized jars, leaving a little space at the top.
Seal the jars tightly.
Cool and Store:
Allow the jars to cool to room temperature.
Store the jam in the refrigerator for up to a month.
