Set your oven to 180C or gas 4. Cut the potatoes into 1cm square chunks, and simmer in salted water until cooked, drain.
Roll out the pastry to a square 30 x 30cm, and place on a floured baking sheet. Next slice the onion and mushrooms. Heat the olive oil in a large frying pan, and gently sauté the onion, mushrooms and the cooked potato for a few minutes until the onions soften.
Add the white wine vinegar and allow to reduce in volume slightly. (This will also de-glaze your pan.) Season with salt and pepper.
Scatter all these things over your puff pastry square and arrange them evenly, leaving a 2cm border. Tear up the cheese and arrange on the veggies.
Bake for about 30-40 mins or until golden brown and bubbly.