Rinse the rhubarb stems with cold water and chop them into half inch bits. Peel and finely chop the onion. Put it all in a pan together with 0.5 cups (1 dl) of the honey, apple cider vinegar, ginger and salt. Let it simmer for 30 minutes. Stir every now and then.
Taste it and see if you want more sweetness. Add more honey if needed.