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.