For the bacon, spring onion and mashed potato cakes, place the potatoes into a pan of salted water, bring to the boil and simmer for 12-15 minutes, or until the potatoes are tender.
Heat a frying pan until hot, add the chopped bacon and cook for 2-3 minutes, or until the bacon is golden-brown and crisp. Remove from the pan and set aside to drain on kitchen paper.
Drain the potatoes and return to the pan. Place the pan over the heat for a few seconds to drive off any excess moisture. Pass the potatoes through a ricer into a bowl and add the butter or olive oil.
Beat the potato mixture with a wooden spoon until well combined, then add the spring onions and cooked, chopped bacon and mix until well combined. Season, to taste, with salt and freshly ground black pepper.
Using your hands, form the mashed potato mixture into four equal-sized patties, then set aside to cool. When completely cool, place the patties into the fridge for 30-45 minutes to chill and set.
Meanwhile, for the sautéed wild mushrooms, preheat the oven to 200C/400F/Gas 6.
Place the bacon onto a baking tray and place a second baking tray on top to weigh it down and prevent it from curling as it cooks.
Cook in the oven for 5-8 minutes, or until browned and cooked through. Remove from the oven and set aside to cool for ten minutes, or until the bacon has crisped up and is firm.
Heat a pan until hot, then add the 50g/2oz butter and one tablespoon of the olive oil. Add the shallot and cook for 2-3 minutes, until the shallot has just softened.
Add the garlic and the mushrooms and cook for another 3-4 minutes, until the mushrooms have released their liquid and are tender. Season, to taste, with salt and freshly ground black pepper.
For the potato cakes, when the potato cakes have chilled and set, remove them from the fridge and dredge in the flour.
Heat a frying pan until hot, add the remaining two tablespoons of olive oil, then add the cakes, two at a time. Cook on both sides, turning carefully, for 3-4 minutes, until golden-brown on both sides and heated through. Keep warm.
Heat a separate frying pan until hot, add the remaining olive oil. Crack in the duck eggs and fry until cooked to your liking.
To serve, place a potato cake in the centre of each plate. Spoon some mushrooms onto each potato cake, piling them up carefully. Top with a fried duck egg, then season, to taste, with a little salt and black pepper. Finish with a sprig of chervil.