We use an 8-inch square broiler-safe baking dish for this recipe, but any broiler-safe dish that holds 2 quarts will work here. At the end of step 3, the base for the pie should look quite thick; it will loosen to the perfect consistency as the seafood cooks. We prefer cold-smoked salmon here because it's less likely to overcook, but you can substitute hot-smoked salmon if you prefer it. Making a pattern on the topping not only looks attractive but also provides textural contrast when the pie is broiled.

FOR THE TOPPING: Place potatoes in large saucepan and add water to just cover. Add salt and bring to boil over high heat. Reduce heat to maintain simmer and cook until tip of paring knife inserted into potato meets no resistance, 8 to 10 minutes. Drain potatoes and return to saucepan over low heat. Cook, shaking saucepan occasionally, until any surface moisture on potatoes has evaporated, about 1 minute. Off heat, mash potatoes well. Stir in butter until melted. Whisk cream and egg yolk together in bowl; stir into potatoes. Season with salt and pepper to taste. Cover to keep warm and set aside.

FOR THE FILLING: Set 8-inch square broiler-safe baking dish on rimmed baking sheet. Sprinkle shrimp with ¼ teaspoon salt and baking soda in bowl and toss to combine. Refrigerate until needed.

Melt 3 tablespoons butter in medium saucepan over medium-low heat. Add leek and thyme and cook, stirring occasionally, until leek is softened, 6 to 7 minutes. Add wine and cook, stirring occasionally, until wine has evaporated, about 5 minutes. Add flour and cook, stirring constantly, for 1 minute. Add clam juice and stir until mixture is smooth. Stir in cream, pepper, and remaining ½ teaspoon salt. Increase heat to medium-high and bring to simmer. Lower heat to maintain simmer and cook, stirring frequently, until mixture resembles thick chowder, 10 to 13 minutes.

Stir cod, salmon, and shrimp into sauce and return to simmer. Cover and cook, stirring every 2 minutes and adjusting heat if needed to maintain simmer, until shrimp are opaque and just cooked through, 4 to 6 minutes. Off heat, stir in parsley. Transfer filling to prepared dish.

Adjust oven rack 8 inches from broiler element and heat broiler. Spoon topping over filling, starting at edges and working toward center. Smooth with rubber spatula, making sure to seal around edges of dish so no seafood or sauce is exposed. Using back of spoon or tines of fork, make pattern on topping. Melt remaining 1 tablespoon butter and drizzle over topping. Broil pie, still on sheet, until topping is golden brown and crusty and filling is bubbly, 6 to 7 minutes (watch closely). Let cool for 10 minutes before serving.

Technique: How to Spread the Spuds