1.Preheat oven to 220°C (200°C fan-forced). Grease a deep 24cm pie dish.
2.Cook pumpkin in a saucepan of boiling, salted water on medium heat 10-15 minutes, until tender. Drain, return to pan and coarsely mash. Season well and stir through reserved tuna oil, eschalot, rice, olives, tomato, lemon rind and juice. Gently fold through tuna.
3.Join two sheets of pastry together by wetting one edge of each and pressing to seal. Line prepared dish, letting excess hang over side. Fill with pumpkin mixture and place remaining pastry sheet on top. Press edges together and trim excess pastry. Make a few slits in top of pastry.
4.Brush with butter and bake for 1 hour. Remove from oven and set aside for 5 minutes. Cut into wedges to serve.