Blend pork belly and fat in a processor, season with fleur de sel and quatre-épices, then chill.
Boil butter, lard, milk, water, and salt, then stir in flour to form a dough.
Remove dough from heat and cool completely.
Roll dough to 2cm thickness and line a 10cm pie mould, reserving some for the lid.
Fill pastry with pork mixture, seal with the lid, and poke a hole in the top.
Glaze with egg yolk and chill for one hour.
Preheat oven to 180°C.
Glaze again and bake for 60 minutes, then remove mould and bake 20 minutes more until crisp.
Dissolve gelatine in boiled apple juice, then mix with remaining juice and cool.
Inject the apple jelly into the cooled pie using a syringe and let it set.
Preheat oven to 220°C.
Fry half the butter with breadcrumbs and mustard until brown, then stir in hazelnuts and apple.
Sear halibut in oil for 30 seconds, then oven-cook for 3-4 minutes.
Preheat the grill to high.
Top halibut with the crust mixture and grill for 2-3 minutes until crisp.
Wilt spring greens in remaining butter and season.
Simmer fish stock and apple juice together.
Blend the prepared pork pie with the stock mixture into a fine purée.
Plate greens and grapefruit, top with halibut, and serve sauce alongside.