Preheat the oven to 200°C, fan 180°C, gas 6. Lay one of the pastry sheets on the work surface, the shortest end facing you. Brush lightly all over with some melted butter, then fold in half. Place the fish at the end nearest to you, season with pepper and spread with 1 tsp of the pesto.
Fold the sides of the pastry over the fish and brush the ends with a little more butter. Fold the remaining pastry over the fish to completely enclose and brush all over with butter. Repeat with the remaining 2 pieces of fish and filo pastry and lay on a baking tray. Bake for 20-25 mins until golden and crisp.
Meanwhile, steam the green beans until tender. Toast the sesame seeds in a dry frying pan and toss the drained beans with the seeds. Serve the beans with the fish parcels, with wedges of lemon to squeeze over.