Preheat the oven to 200°C.
Place mushrooms on a tray, drizzle with olive oil, and season.
Roast for 15 minutes, then reduce heat to 140°C and cook for 90 minutes.
Drain mushrooms and increase oven temperature to 180°C.
Remove sausage meat from skins and place in a bowl.
Mix sausage meat with cheese, shallot, garlic, herbs, mustard, and sauces.
Mould the mixture into four even-sized cakes.
Brown the cakes in a frying pan with olive oil for a few minutes per side.
Transfer cakes to a tray and bake at 180°C for 20 minutes.
Reheat four whole mushrooms in the oven during the final minutes.
Slice the remaining six mushrooms and fry in butter with chives.
Plate the sliced mushrooms, top with a whole mushroom, a sausage cake, and a fried egg.