To Prep: Tear off 8 squares of parchment paper. Arrange 4 on a flat surface. Place the carrots and leeks in the center of each square. Place the halibut on the vegetables. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Drizzle with the oil and sprinkle with the oregano. Top the fish and vegetables with the remaining squares of parchment. Fold over the edges several times to seal. Place the packets in resealable plastic bags and freeze until ready to cook, for up to 3 months.
To Cook: Heat oven to 375° F. Remove the frozen packets from the plastic bags and place them in a single layer on a baking sheet. Cook for 25 minutes. Remove from oven. Transfer each packet to a plate. Using a paring knife, cut open the packets before serving. (Be careful of the escaping steam.)