Place the flour and butter in a food processor and pulse.
Add water a little at a time, just enough to bind the pastry together, and pulse again.
Roll out on a lightly floured pastry board and cut a 27cm (approx) diameter circle.
To make the filling and assemble the galette
Slice the mushrooms and leeks, cut the cherry tomatoes in half and roast in a little olive oil until soft.
Place the pastry circle on a metal baking sheet.
Position the mushrooms, leeks and tomatoes in the centre of a circle leaving a margin of about 3cm around the edge.
Slice the Camembert and cut into pieces and place on top of the vegetables. Add the rocket to the top of the cheese.
Fold the pastry inwards a little at a time, forming pleats to take in the excess pastry, using water to stick the pastry where it folds. Place a few halved tomatoes on the top.
Brush the pastry with beaten egg.
To bake
Bake in the centre of the oven at 180°C, 350°C, gas mark 4 for about 20 to 30 minutes until the pastry is golden and the cheese has melted.