Begin by cooking the potatoes. Place a large pot of salted water over high heat. Add the peeled and diced potatoes and bring to a boil. Cook for 20 to 25 minutes until the potatoes are tender. Drain and set aside.
Meanwhile, prepare the seafood. Peel the shrimp and remove the tails, reserving both to use in the broth. Set the peeled shrimp aside. If using scallops, use a paring knife to remove the small muscle on the side of each scallop. The scallop trimmings can also be reserved to use in the broth. Cut the fish into 1-inch pieces. Transfer all the prepared seafood to a paper towel lined plate to drain and pat dry.
Trim the green tops from the leeks. Set aside four or five 1-inch pieces of the dark green tops to use in the broth. Slice the leek in half and wash between the layers. Thinly slice or dice the leeks and set aside. Trim the stalks from the fennel bulb. Set aside four or five 1-inch pieces of the stalks to use in the broth. Mince the garlic and chop the tarragon.
In a small saucepan, heat the olive oil over medium high heat. When the pan is hot, add the reserved shrimp shells and scallop trimmings (if using). Sauté the shells for 2 to 3 minutes until they are bright pink in colour. Carefully pour in the white wine to deglaze the pan.
Add the reserved leek tops and fennel stalks along with the bay leaves, 1/2 tsp kosher salt and the black peppercorns. Add 1 1/2 cups water and bring to a simmer. Reduce the heat to low and partially cover with a lid that is slightly ajar. Let the broth cook for about 20 minutes on a very gentle simmer. Strain the broth into a measuring cup. You should have about 2 cups of liquid. Add a bit more wine or water to make 2 cups if you have less.
If you intend to pipe out the potato topping, it is recommended to pass the cooked potatoes through a ricer or food mill. This will prevent any lumps which could make piping difficult. Once the potatoes are finely riced, add 3 tablespoons each of the butter and cream along with 1 tsp kosher salt. Blend together until smooth. Add the egg yolks and mix until thoroughly combined. Set aside and keep warm.
Preheat the oven to 400°F. Place a wide deep skillet over medium low heat. Add the remaining 1/2 cup butter along with the leeks and fennel. Sauté the vegetables until the fennel is tender. Add the minced garlic and cook for another minute until fragrant.
Blend in the flour to make a roux. Slowly add the broth in 3 or 4 additions, whisking constantly to prevent lumping. Bring to a simmer and cook until thickened. The sauce will be very thick to ensure the filling won't end up too runny once the seafood is cooked. Stir in the heavy cream along with 1 tsp kosher salt and 1/4 tsp pepper.
Remove from the heat. Add all the prepared seafood and fold evenly into the sauce. Stir in the tarragon. Lightly grease a large gratin dish or a 13 x 9-inch baking dish. Pour the seafood filling into the prepared pan and smooth the top.
Fit a large piping bag with a triangle cut piping tip, such as an Ateco #883 tip With the triangle cut facing upright, carefully pipe out strands of the potato filling, weaving back and forth to create waves and swirls. Cover the seafood filling completely with the mashed potatoes. Alternatively, spoon the mash over the filling to cover completely.
Place the gratin dish on a rimmed baking sheet and bake for 35 to 45 minutes until the potato topping is golden brown, the filling is bubbling, and the seafood is fully cooked.
To serve, sprinkle the top of the pie with finely chopped chives. Enjoy!