Heat 3 teaspoons butter in medium skillet over low heat, add leeks, orange zest, simmer gently until leeks soften, add mirin or white wine, coriander seed and continue to cook until liquid is evaporated, season with salt and pepper. Set aside to cool. Thinly slice washed, unpeeled potatoes (1/4-inch thick) toss slices with melted butter, roast in a single layer on a baking sheet until just tender, approximately 4 minutes. Remove from oven and cool. Combine all ingredients for truffle butter, stir to incorporate. Cut parchment into an elongated heart shape, lay in one layer of potatoes, then 1 teaspoon cooked leeks, then halibut seasoned with salt and pepper, then 1 teaspoon truffle butter. Start at the middle of the heart and fold while creasing well until package is completely enclosed, bake just before serving for 8 minutes.