To make the rough puff pastry, add the flour to the bowl of a Kenwood food processor.
Cut the cold butter into 1/2cm slices and add to the Kenwood food processor bowl.
Using the pulse function process the mixture until the butter is broken up but still in visible lumps. Tip the mixture out into a large mixing bowl.
Make a well in the centre and add the juice of half a lemon and then enough cold water to make a dough. Use the blade of a table knife to mix the dough rather than your hands.
Gather the dough into a ball wrap in clingfilm. Place it into the fridge for an hour until firm.
Remove the dough from the fridge, lightly flour your work surface and then roll out the dough into a long rectangle shape.
Fold into thirds, the top downwards and then the bottom upwards creating an envelope like shape.
Turn the dough 90 degrees to the right so that the folds are now left and right. Roll again to a large rectangle and fold into three again. Turn and then repeat this step twice more, turning before each re-rolling and folding.
Each time the pastry gets smoother and more refined. Wrap again in clingfilm and allow it to chill for another hour in the fridge.
Prepare the filling. Melt the butter in a large frying pan, add the sliced onions and gently fry until softened.
Add the finely chopped garlic, sugar and balsamic vinegar, turn up the heat and fry until the vinegar has evaporated and the onion is caramelised.
Remove from the heat and set aside to cool.
Preheat the oven to 180c(fan)/200c/Gas Mark 5.
Grease or line two baking sheets with non-stick baking paper.
Roll out the chilled pastry on a lightly floured surface into a large rectangle approx. 40 x 30cm. Trim the edges level using a sharp knife or pastry cutter. Divide the sheet into 6 large rectangles.
Working with one piece at a time lightly score two opposite corners with a sharp knife.
Divide the onions between the six pieces of pastry and lay in a length between each scored corner.
Slice the brie into small chunks and divide between the wraps laying them on top of the onions.
Take two opposite corners and fold over the onions and brie to form the wrap shape. Pinch together lightly to seal.
Transfer onto the lined baking sheets, glaze the pastry with a little milk and bake in the oven for 14-16 minutes until the pastry is cooked through and golden.