Melt some butter with a splash of olive oil in a pan, add the onions and a good pinch of salt. Over a very low heat gently saute the onions, covered, and stirring occasionally, for about 15-20 minutes. You might need to add a splash of water to prevent them catching. When golden and sticky, transfer the onions to a plate to cool.

Preheat the oven to 200°C/Gas 6. Roll out the pastry to a rectangle, trim all the edges so you have a neat shape but also because otherwise the pastry will struggle to rise and flake properly. Score a margin round the edge, about a centimetre wide, don't cut all the way through, this will rise and create an edge. Brush the edge with milk.

Put the pastry in the oven and cook for about 10-12 minutes until light golden and puffed up. Remove from the oven and re-cut the inside square with a sharp knife, then push down the centre to create a dip.

Place the cooled onions in the tart case, add some torn black olives and scatter a few capers on top, then grate a little Parmesan over the top. Add lots of black pepper.

Return the tart to the oven and cook for another 5-7 minutes until golden. Allow to cool on a wire rack for a few minutes to keep the base crisp, and then serve warm.