Tip:Much of the excitement of cooking en papillote is the moment when you open the packets and release an enticing cloud of aromatic steam. So that everyone gets to experience that moment, serve the unopened packets on plates and let people open their own.
Heat 1-1/2 Tbs. of the butter in a 3-quart saucepan over medium heat. Add the leek, season with 1/8 tsp. salt and a grind or two of pepper, and cook, stirring frequently, until tender but not browned, 4 to 6 minutes. Add the rice, stir to coat with butter, and cook, stirring until the rice begins to crackle, 2 to 3 minutes. Add 3/4 cup plus 2 Tbs. water, 1/2 tsp. salt, and the lemon zest. Cover and reduce heat to a low. Simmer until the rice is tender and the liquid is absorbed, about 15 minutes. Remove from the heat and let sit, covered, for 5 minutes. Discard the lemon zest, fluff with a fork, and set aside to cool slightly.
Position racks in the upper and lower thirds of the oven and heat the oven to 375°F.
Prepare the parchment packages. Cut four 15x24-inch sheets of parchment. Fold each sheet in half, forming a 15x12-inch rectangle. With a pencil, draw a half-heart on each, centering it on the folded edge. Cut out the hearts. (This shape is easier to seal than a rectangle.)
In a small bowl, mix the remaining 3-1/2 Tbs. butter with 1 Tbs. of the shallot, the parsley, dill, lemon juice, and salt and pepper to taste until evenly combined.
Unfold the parchment and arange on a work surface. Coat the inside of each with butter, leaving a 3-inch border. Distribute the remaining shallots among the sheets on one half of the heart Distribute the rice among the sheets, arranging it in a small mound on top of the shallots. Season the halibut fillets with salt and pepper and put them on top of the rice. Smear the top of the fillets with the seasoned butter, distributing it evenly. Arrange two lemon slices on each fillet and sprinkle the wine over the fish. Fold the other half of the parchment over and line up the edges. Starting at the top of the heart, fold over about 1/2 inch of the edge, pressing down to make a crisp crease. Continue working your way around the edge of the packet, making overlapping folds (like pleats), always pressing firmly and creasing the edge so the folds hold. Twist the tip of the heart to finish. If necessary, make a second fold anyplace that doesn’t appear tightly sealed.
Transfer the packets to two large, rimmed baking sheets and bake for 8 minutes. Rotate and swap the baking sheets’ positions and continue to bake until the packets are puffed and fragrant, about 7 minutes more. Serve immediately.
Serve with a bowl of steamed green beans or a simple tossed salad, if you like.