Method

• 1.
  To make the shellfish butter, preheat the oven to 200°C (180°C fan-forced). Place the shellfish shells, onion, garlic and oil in a medium deep-sided roasting pan. Place in oven and cook for 20 minutes, or until golden brown. Reduce the heat to 120°C (100°C fan-forced), add the melted butter and cook for a further hour, or until the garlic is tender. Remove the soft garlic cloves and set aside. Strain the shellfish mixture through a fine-mesh sieve, reserve the butter and allow to cool.
• 2.
  Place the fish stock in a large saucepan and bring to the boil. Add the marron and poach for 4 minutes. Remove the marron from the stock and refresh in ice-cold water. Set the stock aside.
• 3.
  Using a sharp knife, split the marron lengthways and remove the coral (the orange membranes) from the head section. Set the marron aside, reserving the coral.
• 4.
  To make the thermidor sauce, crush the soft garlic cloves with a fork. Place half the shellfish butter, eschalot and garlic cloves in a large frying pan over medium heat and cook for 1 minute, stirring, or until eschalot is tender. Add the flour and cook, stirring, for 1-2 minutes. Gradually add 400ml of the reserved fish stock, stirring until the mixture is thickened and coats the back of a spoon. Add the reserved coral, gruyere cheese, chervil, chives and pepper, stirring until well combined. Remove from the heat and set aside to cool slightly.
• 5.
  Pre-heat the grill to high. Place the halved marron on a baking tray lined with foil and spoon over the thermidor sauce. Cook for 8 minutes or until the sauce is golden. If you wish, cover the marron (especially the claw) with foil to prevent burning.
• 6.
  Place the marron on a serving plate and lightly dust with the bush tomato powder.