Grate the zest of half of the oranges, making sure you're scraping only the bright orange part of the rind.
Then, peel off and discard the thick white pith for all oranges.
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Now, cut out the orange segments, leaving behind the white membrane that is in between them.
Then, squeeze what's left of the oranges, add the juice to the segments bowl, and discard the membranes.
Add the orange juice, segments, and zest into a pot along with water and erythritol. Then, simmer over low heat for 15 minutes.
Next, blend the jam until smooth and simmer it for 15 more minutes. Then, add the starch dissolved in a little water, and stir until the jam thickens.
Pour the jam into a jar and press the lid down. Then, flip the jar, quickly tighten the lid, and leave the jar upside down for 30 minutes.
If the lid doesn't pop when pressed, you have sealed the jar correctly and you can store it away; otherwise, consume within a week.