In a large heavy-bottomed pan set over medium heat, saute the onions in olive oil, stirring frequently, for 10-15 minutes or until the onions are very soft and golden-brown. Add the sage, salt and pepper, and continue sauteeing for another 2 minutes.

Stir in the blueberries, balsamic, cider vinegar and sugar. Bring to a simmer, then reduce heat to low and cook for 20 minutes or until the mixture starts to thicken and coats the back of a spoon. Remove from heat and spoon into 3 sterilized 250ml jars.

Jam will keep in the fridge for 3-4 weeks. For longer storage, process in a boiling water bath for 10 minutes and store in a cool, dark place for up to 6 months.