Heat 1 tbsp of the olive oil in a frying pan and fry the bacon for 3-4 mins until it begins to crisp. Tip onto a plate lined with kitchen paper. Add the remaining olive oil to the pan with the butter and allow to melt before adding the leeks and thyme. Season and cook for 3-4 mins until softened. Place in a bowl with the crisp bacon and cheese, and allow to cool.
Whisk the egg in a small bowl with 1 tsp water, to make an egg wash. Cut one-third off the pastry and, on a lightly-fl oured surface, roll out to make a 15cm x 30cm rectangle, and place on a baking sheet. This will form the base of the tart. Pile the leek mixture in the middle of the base, leaving a 2cm surround. Roll the remaining pastry into a larger rectangle which will cover the leek mixture easily. Brush the edges of the pastry base with egg wash, then carefully lay over the larger rectangle and smooth the surface to get rid of any trapped air. Cut away excess pastry to form a tidy edge and glaze with egg wash. With the back of a fork, press to seal the edges. Chill for 30 mins.
Heat oven to 200C/fan 180C/gas 6. Brush the top of the pastry with more egg wash and season the top before baking. Cook for 10 mins, then reduce the heat to 180C/fan 160C/gas 4 and cook for 25-30 mins until puffed up and golden brown. Serve either hot or at room temperature.
To make ahead, freeze at end of stage 2 for up to a month, or refrigerate for up to a day.