Arrange the bacon rashers on a baking tray, then place a second baking tray on top. Transfer to the oven and cook for 5-8 minutes, or until the bacon rashers are crisp and golden-browned and cooked through. Set aside to cool for 10 minutes.
Heat a frying pan over a medium heat. Add the butter and, when it is foaming, add the chopped mixed mushrooms and fry for 3-4 minutes, or until golden-brown.
Meanwhile, heat a griddle pan over a medium heat. Drizzle the olive oil over the sourdough bread. Grill the bread slices, in batches, for 1-2 minutes on each side, or until golden-brown griddle marks appear on both sides of the bread. Repeat the process with the remaining slices of sourdough bread. Keep warm.
Meanwhile, heat the vegetable oil in a small frying pan over a high heat. When the oil is hot, crack the truffle-infused duck eggs into the pan, in batches if necessary. Cook for 1-2 minutes, or until the whites are cooked through but the yolk is runny. Remove from the pan and set aside. Repeat the process with the remaining truffle-infused duck eggs. (If using normal duck eggs, add a few drops of truffle oil, if using, to each egg white. Add the egg whites to the hot oil, one at a time, and cook until almost cooked through, then place the egg yolk on top of the white and cook for a further minute, or until the yolk is just warmed through but still runny. Remove from the pan using a slotted spoon and set aside. Repeat the process with the remaining duck whites, duck yolks and truffle oil.)
To serve, place one piece of grilled sourdough bread into the centre of each of four serving plates. Place one truffled duck egg on top of each slice of bread. Place two rashers of crisp bacon alongside each. Spoon the mixed mushrooms alongside, then sprinkle over the chopped parsley.