Preheat the oven to 200C/400F/Gas 6.
For the filling, heat the oil in a small ovenproof frying pan. Add the tomatoes and sugar and fry until the tomatoes just start to soften and the sugar becomes light golden-brown in colour. Scatter the chopped sage over the tomatoes.
For the pastry, place the flour and butter into a food processor. Blend until it resembles breadcrumbs.
Add the oil and water and blend for a few seconds more. Tip out onto a work surface lightly sprinkled with flour and knead lightly to form a soft dough.
Using a rolling pin, roll out to a round circle slightly bigger than the frying pan.
Place the pastry on top of the fried tomatoes and tuck the edges into the filling.
Put the frying pan into the oven and cook for about 12 minutes, until the pastry is golden-brown and cooked on top.
Carefully turn out onto a large plate, so that the tomatoes are on top. Drizzle with balsamic vinegar and serve warm.