Preheat the oven at 190 C fan force.
Chop the bacon and vegetables.
Heat the oil In a fry pan on medium heat. Fry the bacon for a few minutes until the fat has released and the bacon is starting to look cooked.
Add the mushrooms, and fry for a few more minutes or until the moisture has dried up in the mushrooms.
Add the onions and tomatoes, season with salt and pepper then give the mixture a good stir and remove from heat. There's no need to cook the onions or tomatoes, this step is just to warm through and combine with the other ingredients.
Allow the sheet of puff pastry to thaw to the point where it can be easily cut and handled. Using the photos above as a guide, cut into four squares. For each square, cut from the centre of each edge towards the centre, stopping before you get to the middle.