To make the filling, heat the oil in a large non-stick frying pan. Add the bacon, turn the heat to low and gently cook to melt out some of the fat, then turn up the heat to brown a little. Remove with a slotted spoon to a plate and set aside. Add the onions and mace to the pan, and gently cook until softened. Set aside to cool.
Line the base of a 20 x 30cm tin with baking parchment, then add 3 long, thick strips to run across the tin, with overhang to help you lift out the pie. Roll out 750g of the pastry on a lightly floured surface until it’s big enough to line the base and sides of the tin.
Put a baking sheet in the oven and heat to 200C/180C fan/gas 6. Spread the onions onto the base of the pie. Open out each pepper like a book, scrape out any seeds, then layer over the onions to cover. Scatter over the bacon, followed by the parsley. Evenly crack over the eggs, and season all over.
Roll out the remaining pastry to cover, pinch and trim the edges to seal, and brush with beaten egg. Scatter with sesame seeds. Sit on top of the baking sheet and bake for 30-40 mins until golden and crisp. Cool at room temperature, then store in the fridge for up to a day. Bring just back to room temp before serving in squares.