How to Make It
Arrange the halibut in a shallow baking dish. In a medium saucepan, combine 1 cup of water with the sugar, fennel seeds, peppercorns, coriander seeds and 2 1/2 tablespoons of kosher salt and bring to a simmer. Cook, stirring, until the salt and sugar dissolve. Remove the pan from the heat and stir in 3 cups of ice water. Pour the cold brine over the halibut and let stand at room temperature for 25 minutes. Remove the halibut from the brine and pat dry; discard the brine. Season the halibut lightly on one side with salt and pepper.
Preheat the oven to 400°. Arrange a 12-by-32-inch piece of parchment paper on a baking sheet. Mound the corn kernels on one half of the parchment paper and top with the fish, skinless side up, leaving a 3-inch border on 3 sides. Top with the beans and tomatoes, then drizzle with the oil and vinegar. Scatter the tarragon and thyme on top. Set the reserved corn cob next to the fish. Top the fish with the cubed butter. Fold the parchment over the fish and pleat, fold and crimp the edges to seal; add the water before completely sealing the packet.
Roast for 20 minutes, until the halibut is just opaque throughout. Transfer the packet to a serving platter, open and serve.