I found this on the Cook's Illustrated website when I was looking for recipes to use some of the dill I've got growing. It's very good. I love dill but even so I don't recommend exceeding the 1/4 cup amount.
Unlike most pot pies, this one does not have the conventional top pastry crust because it would cause the fish beneath it to overcook. Our topping of choice here is puff-pastry cut-outs, which are easy to make and attractive. The flavor of fresh dill is important here, so do not substitute dried.
Flour for the counter
1 sheet frozen puff pastry (9 1/2- by 9-inch), thawed
1 large egg , lightly beaten
2 pounds salmon fillets , skin removed
Table salt and ground black pepper
6 tablespoons unsalted butter (3/4 stick)
3 leeks , white and light green parts only, halved and sliced 1/4 inch thick
2 cloves garlic , minced
7 tablespoons all-purpose flour
2 (8-ounce) bottles clam juice
2 cups whole milk
1 cup frozen peas
1/4 cup minced fresh dill
2 tablespoons fresh lemon juice
Pinch cayenne pepper
Lemon wedges (for serving)
See Illustrations Below: Making Puff-Pastry Rectangles
1. For the topping: Adjust an oven rack to the lowest position and heat the oven to 425 degrees. Dust the counter lightly with flour and unfold the sheet of puff pastry. Following the photo, use a pizza cutter to cut the pastry into 12 rectangles. Brush the rectangles lightly with egg, then transfer to a baking sheet lined with parchment paper. Bake until the rectangles are puffed and lightly browned, about 8 minutes. Allow to cool on the baking sheet until needed.
2. For the filling: Reduce the oven temperature to 400 degrees. Remove any pin bones from the salmon and cut into 1/2-inch pieces. Season the salmon with salt and pepper, then spread it in a 9 by 13-inch baking dish and set aside.
3. Melt the butter in a large Dutch oven over medium heat. Add the leeks and 1/2 teaspoon salt and cook until softened, about 5 minutes. Stir in the garlic and cook until fragrant, about 15 seconds. Stir in the flour and coat the vegetables. Gradually whisk in the clam juice and milk until smooth. Bring to a simmer and cook until the sauce is thick, about 1 minute.
4. Off the heat, stir in the peas, dill, lemon juice, nutmeg, and cayenne. Season with salt and pepper to taste. Pour the mixture over the salmon in the baking dish. Use a spoon to redistribute the salmon evenly throughout the dish.
5. Arrange the reserved puff-pastry rectangles attractively over the casserole. Bake until the sauce is bubbly and the salmon is fully cooked, 13 to 15 minutes. Let sit for 5 minutes before serving with the lemon wedges.
To Make Ahead
The topping in step 1 and the sauce in step 3 may be prepared up to 1 day in advance. Wrap the pastry squares tightly in plastic wrap and store at room temperature. Transfer the sauce to a covered container and refrigerate. When ready to assemble and bake the casserole, return the filling to a simmer in a saucepan or the microwave before proceeding with steps 2, 4, and 5.
STEP BY STEP: Making Puff-Pastry Rectangles