Put the flour and butter in a bowl and rub together with your fingertips until the mixture resembles breadcrumbs. Stir in the Parmesan and enough egg yolk to bring the mixture together into a dough.
Roll out the pastry on a lightly floured surface, brush with the remaining egg and sprinkle over the sesame seeds. Cut the dough into about 20 biscuits with a biscuit cutter, transfer to nonstick baking sheets and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for about 10 minutes. Leave to cool on a wire rack.
Put the trout, capers, crème fraîche, horseradish and dill in a food processor or blender and process for about 10 seconds until the ingredients are combined but still retain some texture. Chill for about 30 minutes, then serve with the biscuits. Uneaten biscuits can be stored in an airtight tin for a few days.