For the shortcrust
200 g flour
1 teaspoon of salt
90 g cold butter
3 tablespoon cold water
1 egg yolk
For the filling
1 tablespoon olive oil
2 red onions
1 tablespoon balsamic vinegar
1 tablespoon sugar
800g medium size tomato halved across the middle
1 tablespoon fresh thyme leaves
1 tablespoon dried oregano
1 tablespoon whole grain mustard
Mix flour, salt and butter until fine crumbs form. Add the water and mix until getting a dough. Wrap it in a clingfilm and let it sit in the fridge at least 30 minutes.
Add the tomatoes, skin-side down, in a try. Put in preheated oven at 180C for 30 minutes until they release their juices.
Chop the onions and fry them in medium heat frying pan with the olive oil for 5 minutes. Add the balsamic and the sugar. Let it cook 5 more minutes without mixing. Add the oregano and thyme, mix. Set aside.
Lift out the tomatoes with a slotted spoon and rearrange in a tart tin, skin-side down. Cram them in as they will shrink a little and you don’t want any gaps.
Add the onions mixture and the mustard.
Roll the pastry out on a lightly floured surface to a 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.
Serve warm or cold with a side salad.