On a frying pan heat oil and 1 tbsp olive oil and 1 tbsp butter, add the onions and garlic, fry on medium high heat for 7-10 minutes until golden brown; set aside.
On the same frying pan melt 1 tbsp butter, add the mushrooms and fry on high heat for approx. 15 minutes stirring from time to time, until all the liquid evaporates.
Add the fried onions and garlic, season well and cook for 3-5 minutes longer (there mustn’t be any liquid), leave to cool down.
Beat 2 eggs and truffle oil and add to the mushroom mixture together with the breadcrumbs; season well and stir.
Boil 8 eggs for 7 minutes. Cool in cold water, peel and set aside.
Prepare the baking dish: melt 10g butter and brush it over a rectangular tin (not bigger than 9cm x 30cm), then dust with flour. Cut a piece of baking parchment to fit the bottom and long sides of the baking dish (there should be some extra paper hanging over the edges so you can use it later to help you take the pie out from the dish).
Put the flour with 2 tsp salt into a bowl.
Put the butter, milk and water into a pan, heat until the butter melted. Pour into the flour and mix with a wooden spoon until combined.
Transfer onto the surface and knead until everything comes together.
Heat an oven to 200C (fan ovens to 180C).
Take about two-thirds of the pastry dough and shape it (with your hands or a rolling pin) into a rectangle approx. 12 -15 cm wider and longer than the width and length of the bottom of the dish.
Transfer it into the baking dish and, using your fingers, press it into the bottom, the corners and up the sides of the dish until it comes to the top and hangs over the rim a little.
Place half of the mushroom mixture at the bottom of the dish. Use your fingers or a spoon to make a trench down the middle; place the eggs there. (If you want to make sure each slice of the pie will include egg yolk, cut approx. 1 cm from the tops and bottoms of the eggs).
Put the rest of the mushroom mixture on top of the eggs; spread it and press to level it.
Brush the overhang of the pastry with the beaten egg, then roll out the rest of the pastry to fit over the top of the pie.
Pinch the edges of the pastry together.
Brush the top with the beaten egg and pierce three holes along the top.
Place the pie in the hot oven and bake for 30 mins.
Lower the heat to 180C (160C for fan ovens), and bake for further 30 minutes. Leave to cool.
Use the sides of the paper to pull the pie out of the baking dish. (If its sides are too pale, you can return the pie to the oven set at 200C (or 180C for fan ovens) until browned.
Keep in the fridge but take it out at least 30 minutes before serving.