Preheat the oven to 375 degrees F. Place 4 individual gratin dishes on sheet pans. (If recipe is doubled serve it in a 14 by 9 1/4-inch oval gratin dish.)
Combine the stock, cream, 1/2 cup of the wine, the tomato puree, and saffron in a medium saucepan. Bring to a boil, lower the heat, and add the shrimp. After 3 minutes, use a slotted spoon to remove the shrimp 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.
Continue to cook the sauce until reduced by half, about 12 minutes. Mash 1 tablespoon of the butter together with the flour. Whisk the butter mixture into the sauce along with 1 teaspoon salt and 1/2 teaspoon pepper. Simmer, stirring constantly, until thickened, about 5 minutes. Set aside.
Melt 3 tablespoons 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 tablespoons of wine, 1/2 teaspoon salt, and 1/4 teaspoon pepper and cook for 5 to 10 minutes, until tender. Set aside.
Combine the panko, Parmesan, parsley, tarragon, and garlic. Melt the remaining 3 tablespoons of butter and mix it into the crumbs until they're moistened.
Divide the seafood among the 4 gratin dishes. 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.
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