For the pork pie, blend the pork belly and pork fat in a food processor until coarsely chopped. Season well with the fleur de sel and a pinch of quatre-épices powder. Chill in the fridge.
Bring the butter, lard, milk, water and salt to the boil in a saucepan, then stir in the flour to form a dough.
Remove the pan from the heat and set aside to cool completely.
Roll the dough out to a 2cm/¾in thickness and line a 10cm/4in pork pie mould with the pastry, reserving some of the pastry for the lid.
Spoon the pork mixture into the pastry, ensuring there is no trapped air. Cover the pie with reserved pastry and seal it all the way around with your index finger and thumb. Poke a small hole in the top of the pie.
Glaze the pie with egg yolk and chill in the fridge for one hour.
Preheat the oven to 180C/350F/Gas 4.
Glaze the pie again with egg yolk and bake for one hour. Remove the mould and bake for a further 20 minutes, or until the pastry is crisp. Remove from the oven and set aside to cool completely.
Bring a small amount of the apple juice to the boil, then stir in the gelatine until it has dissolved. Add the remaining apple juice and set aside to cool slightly
Fill a food grade plastic syringe with the jelly mixture and inject it all over the pie. Set aside to cool completely.
For the halibut, preheat the oven to 220C/450F/Gas 8.
Heat a frying pan until hot, add half of the butter and all the breadcrumbs and mustard powder and fry until golden-brown. Stir in the hazelnuts and apple and cook for a further 2-3 minutes, then set aside.
Meanwhile, heat a separate ovenproof frying pan until hot, add a little oil and fry the halibut for 30 seconds. Turn the halibut fillets over and cook in the oven for 3-4 minutes, or until just cooked through.
Preheat the grill to high.
Place the halibut fillets onto a baking tray and sprinkle over the breadcrumb mixture. Place under the grill for 2-3 minutes, or until crisp.
Heat another frying pan until hot, add the remaining butter and the spring greens and cook for 2-3 minutes, or until just wilted. Season with salt and freshly ground black pepper.
For the pork pie sauce, heat the fish stock and apple juice until just simmering.
Blend the pork pie and apple juice mixture in a blender to a fine purée.
To serve, spoon the spring greens onto serving plates, top with a few pieces of pink grapefruit, then place the halibut on top. Spoon the sauce alongside.