Step 1

Roll out the puff pastry on a lightly floured worksurface to 3-4mm thick, then use a roughly 24cm diameter plate as a template to cut the pastry into a circle. Put the pastry disc in the fridge to chill until needed.

Step 2

Put a large ovenproof frying pan over a high heat and add 25g of the butter. Once it begins to foam, put the onions in the pan cut-side down and season. Cook for 4-5 minutes until the flat surfaces of the onions are well browned. Remove from the pan and cool. Once cool, remove any dry layers of onion/skin and discard. Keeping a small piece of root attached, slice each onion half lengthways into 2 wedges.

Step 3

Heat the oven to 170°C fan/gas Clean the frying pan, then put it over a medium heat. Once hot, add the caster sugar and cook slowly for 5 minutes, swirling it gently if necessary to help it melt but not stirring, until you have a deep amber caramel. Just before it begins to smoke, turn off the heat, then deglaze the pan with the vinegar and Worcestershire sauce. Keep stirring gently. Once the liquids are incorporated, add the remaining cubed butter and whisk until emulsified. You may find it easier to add the butter in small batches to keep the caramel from splitting. Allow the caramel to set for 5 minutes.

Step 4

Once the caramel has set, scatter the star anise and thyme sprigs evenly over it. Arrange the onion quarters on top of the caramel in a circle around the edge of the pan, then in the centre of the pan in the same pattern. Keep the onion quarters as close together as possible as they will shrink during cooking. Remove the puff pastry disc from the fridge and place it over the arranged onions, using a spoon to tuck the pastry in and around the inside edges of the frying pan. Make 6 tiny cuts evenly over the top of the pastry – this will allow any steam to escape while the tarte is baking.

Step 5

Bake in the oven for 35-40 minutes until the pastry is evenly risen and golden brown. Remove from the oven and allow to cool for 10 minutes.

Step 6

While the tarte cools, put the ingredients for the cheddar mascarpone in a food processor or blender and whizz for 30 seconds until you have a thick, smooth cream.

Step 7

To finish the tarte, run a small knife around the inside of the pan to ease away the pastry. Place a large serving dish on top of the pan, then quickly turn the pan upside down, releasing the tarte (this needs to be done swiftly to prevent any liquid running out). Scatter over some extra thyme, then serve in slices with the cheddar mascarpone.