Prepare the baking dish or pan. Preheat an oven to 400°F / 205°C. Grease a 3.5 QT pan (or larger cast iron skillet) with nonstick cooking spray, then set aside.
Sauté the leeks. In a medium saucepan, melt 2 Tbsp butter over medium to medium-high heat. Add thinly sliced leeks (or shallots), then sauté until softened and translucent, about 5-7 minutes. Add 2 cloves chopped garlic, then cook 1 minute more. Pour into the greased pan or skillet.
Warm the stock and cream. Combine 1½ cups fish stock with 1 cup heavy cream in a saucepan, then bring to a gentle simmer.
Make the cream sauce. Meanwhile, melt 4 Tbsp butter in the other saucepan (that you sautéed the leeks in), then whisk in ¼ cup all-purpose flour. Cook, stirring constantly, for about 2-3 minutes, then whisk in the warm stock and cream mixture. Continue cooking over medium-high heat until you have a smooth, thick sauce, about 2 minutes more.
Season the sauce. Add 2 Tbsp tarragon, 2 Tbsp parsley, 2 tsp Dijon mustard, 1 tsp fresh lemon zest, 1 tsp Kosher salt, and ¼ tsp black pepper. Stir in 8oz frozen peas until well mixed. Allow to cool slightly while you assemble the rest of the dish.
Add the seafood. Arrange the seafood on top of the sautéed leeks in a single layer. Pour the slightly cooled cream sauce on top.
Add the phyllo dough. Melt the remaining 2 Tbsp butter. On a large cutting board or other clean work surface, place 1 sheet of phyllo dough down, then brush a tiny bit of butter on top, just enough to adhere a second sheet of phyllo to it. Place the second sheet of phyllo on top, then brush with a tiny bit more butter. Use a sharp knife to cut into 12 equal squares, about 2-inch wide. Repeat with remaining 2 sheets of phyllo.
Add the phyllo dough. Place each square of phyllo dough on top of the fish pie, crumbling it to fold and point the end pieces up (it'll kind of look like a flower). Cover the entire pie with phyllo. Doesn't have to be perfectly symmetrical!
Cook the fish pie. Cook for 25-30 minutes, or until the filling is piping hot and the phyllo is golden brown and crispy. Serve immediately!