Preheat the oven to 400°. Lightly oil a baking dish. On a work surface, overlap 3 of the grape leaves, vein side up, to create a 7- to 8-inch round. Repeat with the remaining grape leaves for a total of 4 rounds. Drizzle 1 teaspoon of the olive oil into the center of each round; sprinkle 1 teaspoon of the thyme over all of the rounds. Season with salt and white pepper. Set the halibut on the seasoned oil and fold the grape leaves around the fish. Arrange the fish packets, seam side down, in the baking dish. Roast for about 20 minutes, or until the grape leaves are crisp and the fish is just cooked through.