For the mushroom mix:
first soak the ceps in hot water (enough to cover) for around 15 minutes. Strain the liquid and reserve. Chop the ceps finely.
Heat the butter in a pan until nut brown and it smells ‘toasty’. Add the mushrooms. Sauté for 10 minutes, then add the soaking liquid. Add a pinch of salt and a little truffle oil and sweat on a medium heat until the moisture has evaporated. Remove from the heat and allow to cool. Stir in the Parmesan then check the seasoning and adjust with salt and lemon juice, to taste.
For the celeriac purée:
Melt the butter in a pan, then add the celeriac and a pinch of salt. Sweat on a medium heat until soft but not completely cooked. In a separate pan, heat the milk to a gentle simmer, then pour over the celeriac and simmer for 2 minutes. Tip into a blender and blitz to a smooth purée. Pass through a sieve and allow to cool. Refrigerate until required.
To make the crisps:
Blanch the cabbage leaves then pat dry and leave to cool. Turn the oven on to its lowest temperature. Arrange the leaves on a baking sheet and leave overnight until they have dehydrated and become crisp.
For the pastry cases (shells)
Mix the flour and butter together until they resemble breadcrumbs. Add the salt and cream and knead until you have a pliable dough. Wrap in cling lm (plastic wrap) and refrigerate for at least 30 minutes. Preheat the oven to 180oC/350oF. Remove the pastry from the refrigerator and roll out to 5 mm/¼ inch thick. Line 6 x 8 cm/3¼ inch non-stick tart cases with the pastry and bake for 25 minutes, until golden.
About 1 hour before serving, make the celeriac foam. Whisk the cold ingredients together with the celeriac purée until smooth. Pour into an iSi whipper and t with two N20 cream chargers. Sit in a 65°C/150°F water bath for 1 hour, shaking every now and then to equalise the temperature.
Preheat the oven to 80°C/175°F. Warm the mushroom mixture, loosening with a little cream if needed. Divide among the cases. Make a dent and carefully add the yolks. Bake for 10 minutes, then remove from the oven. While the tarts are baking, deep-fry the dehydrated cabbage leaves until crisp. Drain well on paper towels then crumble into rough pieces.
To serve, lift the tarts onto plates, then cover evenly with celeriac foam. Dust the surface of each tart with cep powder and finish with cabbage crisps.
To make the cep powder:
Arrange the ceps on a baking sheet and warm in a 150°C/300°F oven for 5 minutes. Transfer to a coffee grinder and blitz to a powder. Store in an airtight container until ready to serve.