Heat oven to 220C/200C fan/gas 7. Melt the butter with a splash of olive oil in your widest frying pan. Add the tomatoes, skin-side down, in a single layer (you can do this in 2 batches if they won’t all fit) and cook over a low heat until they release their juices. Lift out with a slotted spoon and rearrange in a tart tin (roughly 23cm), skin-side down – cram them in as they will shrink a little and you don’t want any gaps. Add the sugar and vinegar to the pan, and cook until the pan juices are reduced and syrupy. Drizzle over the tomatoes in the tin, then scatter with the oregano or thyme and season.
Roll the pastry out on a lightly floured surface to a good 25-26cm round. Lay on top of the tomatoes, and tuck the edges down. Use a fork to prick holes all over the pastry – this will help the steam to escape.
Sit the tart tin on a flat baking tray and bake for 30 mins or until the pastry is golden brown and crisp. Let the tart sit for 10 mins, then run a knife round the edge to release the pastry. Carefully flip the tart over onto a serving plate or board and scatter with more herbs to serve.