Heat butter and olive oil in a large pan on a medium heat, add onions and cook gently for 20 to 30 minutes until onions are soft and golden.
Add garlic and cook for 1 minute. Add sugar, mustard, rosemary, bay leaves, and cinnamon sticks. Stir until melted then simmer uncovered, stirring occasionally until mixture is reduced and caramelized. Add vinegar and simmer uncovered for about 5 minutes until thickened slightly. Stir in the rosemary and mustard seeds.
Remove the cinnamon sticks and bay leaves. Spoon the delicious onion jam into a sterilised jar. Eat immediately or keep in the fridge for up to 3 weeks.
Makes a 500gm jar.