12 large brined grape leaves, rinsed and stems trimmed
1/4 cup plus 4 teaspoons extra-virgin olive oil
2 teaspoons chopped thyme
Kosher salt and freshly ground white pepper
Four 6-ounce skinless halibut fillets, about 1 inch thick
3 tablespoons fresh lemon juice
1 teaspoon Dijon mustard
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.
Meanwhile, in a small bowl, whisk the lemon juice with the mustard and the remaining 1/4 cup of olive oil and 1 teaspoon of thyme. Season the vinaigrette with salt and white pepper.
Transfer the fish packets to a platter. Carefully peel back the grape leaves. Spoon the vinaigrette over the fish and serve hot or at room temperature.
The fish can be kept at room temperature for 1 hour.