First make the onion jam. Heat the olive oil and the butter in a small frying pan over a low heat for a few minutes until the butter has melted.
Thinly slice the onions and add to the pan with the salt. Allow them to caramelise in the pan for about 10-15 minutes, stirring frequently so the onions don’t burn.
Once the onions are golden in colour, add the brown sugar, balsamic vinegar and Worcestershire sauce and heat for a further 5-10 minutes until the sauce has thickened. Taste for seasoning.
Remove from the heat and decant into a sterilised jar if you're not going to use it immediately. It should keep for up to 2 months in the fridge, but if you’re like me it never lasts that long.
For the tarte tatin, preheat the oven to 180°C/160°C fan/gas 4.
Add the butter, caster sugar, onion jam, balsamic vinegar, honey and thyme leaves to a saucepan over a medium heat. Cook for about 10-15 minutes or until the sauce has reduced and become thick, stirring regularly so the mixture doesn’t catch on the bottom of the pan. Season with salt and pepper and remove from the heat.
Take a 20cm tart tin or frying pan and use it to cut out a circle from the sheet of puff pastry. You can put the pastry on a plate and put it in the freezer for a few minutes to make it easier to work with.
Add the sauce to the tart tin or saucepan and place the halved plums on top of the sauce, cut side facing down. Place the pastry over the top of the plums and tuck it in round the sides of the tart pan.
Put in the centre of the oven and cook for about 30 minutes or until the pastry is golden and cooked through.
Remove the tart from the oven, and allow to cool slightly. Run a knife around the edge of the pastry to make it easier to remove from the pan. Place a large serving plate over the top of the tart tin and quickly turn it over so the tin is on top and the tart is on the plate. Take care when doing this as the tin will still be hot.
Add the slices of goat’s cheese to the tart while it is warm so it melts. Garnish with the chopped parsley.
The tart can be enjoyed straight away but is just as good cold.