Preheat oven to 400 F. In a medium saucepan, place cod and pour in milk. Slowly heat until milk is steaming and fish is poached, about 5 to 7 minutes. Remove cod and separate into small pieces, removing any skin. Set aside.
In a medium skillet, melt 2 tablespoons of butter. Add sliced leeks and sauté for about 5 minutes. Stir in flour for 1 minute, or until it begins to smell nutty. Gradually whisk in horseradish and milk from poaching. Continue to whisk as sauce thickens for a few minutes. Add peas, dill, lemon zest, salt, pepper, cod, and flaked smoked trout, stirring to combine. Divide into 4 individual ramekins.
Use a piping bag or spatula to spread mashed potatoes onto the top of each ramekin, then brush with melted butter. Place ramekins onto a baking sheet to avoid dripping and bake for 15 to 20 minutes, until golden and bubbling. Garnish with dill sprigs.
Per serving: 522 calories, 21 grams fat (10 grams saturated fat), 45 grams total carbohydrates, 4 grams fiber, 39 grams protein.
