Heat the oil in a medium sauce pan on medium heat. When the oil is hot add the onions and cook until they’re soft and slightly brown around the edges. Approximately 4 to 5 minutes. Add the apples and stir to combine with the onions.
Add the water and apple cider vinegar and bring to a boil. When boiling reduce the heat to a low simmer and add the curry, cumin, ginger, mustard powder, molasses, and sea salt. Stir well to combine. Simmer for thirty minutes (add a small amount of water if needed to avoid scorching) or until the apples begin to break apart and can easily be mashed. Remove from heat and cool.
With an immersion blender or a potato masher, mash the apple mixture until it resembles relish.
Store in the refrigerator in an air tight container for 5 to 7 days.
Enjoy!