To make the bread, place all the ingredients into a bowl and mix together until a dough forms. Flour a work surface and place the dough on it. Knead for 2 minutes and cover with a damp clean tea towel while you make the topping.
For the topping, heat a medium-sized pan over a medium heat and add the oil and butter. When the butter starts to bubble, add the onion, garlic and rosemary. Cook for 5 minutes, stirring often until the vegetables are beginning to soften. Lower the heat and gently cook for 5 more minutes and then add the vinegar and sugar. Cook until the vinegar has reduced to nothing and then add the capers. Remove the pan from the heat and spoon the mixture onto a tray to cool.
Heat your oven to 220C/425F/gas mark 7 and oil 2 baking sheets with olive oil.
While the mixture is cooling, flour a work surface and divide the bread dough into 8 equal balls. Roll out the dough to the thickness of a £1 coin, then lay each piece onto the oiled trays. Add the basil into the cooled onion mixture, then spread it on each of the rolled-out dough pieces, right to the edges. Place each flatbread into the oven and cook for 18 minutes.
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