Heat the butter and olive oil in a large enamel pot on medium heat.
Add the onions and caramelise over low heat without browning the onions.
Add the mustard seeds, chilli flakes, sugar and salt & pepper and mix well.
Let the onions cook over low heat, covered with the lid for an hour, stirring often. Be careful not to let the mixture burn.
Add the rooibos tea and both types of vinegar and let the onion mixture cook for another hour, uncovered. Keep on stirring and cooking until you have a thick jam-like consistency.
While the jam is cooking, wash the jars and lids in warm soapy water and rinse them in boiling water from a kettle - I do this in the sink. Let the jars and lids dry upside down on a clean kitchen towel until needed.
Once the jam is done, spoon it into the jars while still hot. Fill the jars to the top and close tightly. Wipe down the jars with a damp cloth to clean off any spillage.
Jars can be kept in a cool food cupboard for about 4 weeks. Store in the fridge once opened.