Start by peeling and cleaning the onions. Once they're clean, chop them into large pieces and then finely mince them using a food processor.

Place the minced onions in a suitable pot. Add sugar and freshly squeezed lemon juice, then mix everything together. Cover the pot and let it sit for about 3 hours, allowing the sugar to draw out the natural juices from the onions.

While the onions sit, wash the apples to remove any dirt or residue. Peel and core the apples, then grate them using a coarse grater. Mix the grated apples with the apple vinegar and set them aside.

Put the pot with the onions on medium-low heat, allowing the sugar to dissolve and the mixture to gently boil. Let it boil for 5 minutes, then add the apple-vinegar mixture.

Mix everything carefully together and bring it to a gentle boil again. Cover with a lid and let it boil for an additional 15 minutes.

Remove the pot from the heat. Keep the pot covered and let it cool for 4-8 hours or overnight, allowing the apples and onions to soak up the sugary syrup and caramelize.

After cooling, add water to the apple-onion jam mixture.  Mix everything together. Then, bring the mixture back to a gentle boil.

Once the mixture is boiling, reduce the heat to low, cover with a lid, and let the apple onion jam simmer for 1.5 hours.

Remove the lid and add all the grapeseed oil. Carefully mix the onion jam mixture with the oil and continue cooking on the lowest temperature setting for another 30 minutes. By the end of this cooking time, the jam will resemble more of a solid marmalade than a jam. You'll need to pack it into jars rather than pour it.

Once the jam is ready, pack it into jars and process it according to your preferred canning method.