Place a heavy sheet pan in the middle rack of your oven, and preheat oven to 400°F / 200 °C.
Butter a 1.5 qt casserole dish. Make sure to butter the top edges as well, since the puff pastry will puff up over the edge.
Lightly dust a work surface to roll out the puff pastry. Roll out about 14-15 oz of the puff pastry on the floured surface in a rectangle shape (to fit the bottom of the casserole dish), that’s about 2 mm in thickness. If using puff pastry sheets, place two sheets slightly overlapping each other, and roll them out until they are about 2 mm thick (and the two sheets are joined together). This will be the bottom pastry.
Roll out the remaining puff pastry (or 1 sheet of puff pastry) to fit the top of the pie dish, about 3 mm in thickness. Cover with a napkin/plastic wrap and set aside.
Place the sheet of pastry in the casserole dish, and gently fit it on the bottom of the dish, with the pastry hanging over the edges. Be careful not to stretch it too much as this can result in shrinking. Using a fork, dock the bottom of the pastry.
Place about ⅓ of the bacon on the bottom of the pie.
Break 2 eggs into the pie dish and using a fork, break the egg yolks and spread the eggs on the base of the pie dish. Break an extra egg, but reserve the egg yolk in a separate bowl, add the egg white to the pie.
Break in the rest of the eggs into the pie, while evenly spacing them through the dish. Sprinkle some salt and pepper on top.
Dollop ½ of the leek mixture over the eggs, followed by the rest of the bacon, then the rest of the leeks and finally the grated cheese. Sprinkle a little salt and pepper over the cheese.
Brush some water around the edge of the pie, and place the top pastry to cover the casserole dish. Press the edges gently to seal. Roll the excess to create a ridge along the edge of the pie, and crimp it using a fork to completely seal the edges of the pie.
Mix the extra egg yolk with about 1 tbsp of water and whisk to combine. Brush this egg yolk over the surface of the pie. Cut extra decorations (optional), and place them on top of the pie. Brush the decorations with the egg wash as well. Make 5 slits in the pie carefully (to avoid piercing one of the eggs).
Place the pie in the preheated oven, on top of the preheated baking tray. Lower the heat to 375°F/190°C, and cook for about 50 minutes.
Check the pie after 20 minutes, rotate the pie if necessary and check every 10 minutes afterwards. If the top looks like it’s going to burn, cover with foil.
When the pie is baked, remove it from the oven and let it cool down. Slice and serve while warm or at room temperature.
Serve with tomato ketchup or tomato relish.