2) For the sauce, combine the stock, cream, 125ml of the wine, the tomato puree, and saffron in a medium saucepan. Bring to the boil, lower the heat, and add the prawns.
3) After 3 minutes, use a slotted spoon to remove the prawns to a bowl. Add the halibut to the stock for 3 minutes, until just cooked through, and remove to the same bowl. Add the cooked lobster to the bowl.
4) Continue to cook the sauce until reduced by half, about 12 minutes. Mash 1 tbsp of the butter together with the flour. Whisk the butter mixture into the sauce along with 1 tsp salt and 1/2 tsp pepper. Simmer, stirring constantly, until thickened, about 5 minutes. Set aside.
5) Melt 45g of the butter in a medium saute pan. Add the leeks and carrots and cook over medium heat for 5 minutes, until softened. Add the remaining 3 tbsp of wine, 1/2 tsp salt and 1/4 tsp pepper and cook for 5 to 10 minutes, until tender. Set aside.
6) Combine the panko, Parmesan, parsley, tarragon and garlic. Melt the remaining butter and mix it into the crumbs until they're moistened. Divide the seafood among the 4 gratin dishes.
7) Strew the vegetables on top of each dish. Pour the sauce equally over the seafood and vegetables and spoon the crumbs evenly on top. Bake for 20 minutes, until the top is browned and the sauce is bubbly. Serve hot.