Finely dice the onion. Finely chop the ginger. Remove the ends from the rhubarb and cut into small pieces.
Place the sugar, onion, ginger, mustard seeds and cinnamon powder in a saucepan and cook over a medium heat until lightly caramelised. Add the rhubarb and deglaze with cider vinegar. Let it boil down for about 15 minutes without a lid.
Fill into clean jars while still hot. The chutney will keep for several weeks in the fridge, but can also be frozen.
Notes
If you like your chutney a bit chunkier, add only 2/3 of the rhubarb directly. The rest goes into the pot 5 minutes before the end of the cooking time. Or simply cut the rhubarb into larger pieces next time.