Roll the pastry out on a lightly floured surface and use to line a 20cm round tart tin. Chill in the fridge for 20 minutes. Preheat the oven to 200°C/gas mark 6.
Remove the pastry case from the fridge, top with a sheet of baking parchment and fill with baking beans or uncooked rice. Bake for 10 minutes. Remove the beans and baking parchment and cook for another 5 minutes.
Meanwhile, heat the butter in a pan. Add the leek and cook, stirring occasionally, for 5-8 minutes until softened. Stir in the peas and remove from the heat. Lower the oven temperature to 180°C/gas mark 4. Mix together the crème fraîche, eggs and milk and season.
Tip the leek and peas into the bottom of the partly cooked pastry case and spread evenly, then top with pieces of smoked trout. Carefully pour over the egg mixture, then bake for 25-30 minutes, until the filling has set. Allow to cool slightly before garnishing with rocket leaves.