Heat 2 tsp of oil in a large saucepan over medium heat. Cook lamb, turning occasionally, for 5 mins or until brown all over. Add onion and cook, stirring, for 2 mins or until lightly browned. Add the wine or stock and tomato paste and bring to the boil. Reduce heat to low. Cover and cook for 2 hours or until lamb is falling off the bone. Transfer the lamb to a heatproof bowl. Set aside to cool slightly. Use 2 forks to coarsely shred the meat. Discard the bones. Return the meat to the pan and stir to combine. Set aside to cool slightly.
Meanwhile, rinse the silverbeet leaves and drain. Place in a saucepan over high heat. Cook, covered, turning occasionally, for 5 mins or until the silverbeet wilts. Place in a colander. Set aside to cool. Use your hands to squeeze out excess moisture from silverbeet. Coarsely chop.
Combine silverbeet, ricotta, fetta, spring onion, egg, half the dill and half the mint in a large bowl. Season.
Preheat oven to 180°C. Place pastry sheets on a clean work surface and cover with a damp tea towel. Brush 1 sheet with melted butter. Top with another sheet. Continue layering with butter and 6 more pastry sheets. Line a 15cm x 20cm (base measurement) baking pan with the pastry, allowing sides to overhang. Spoon the silverbeet mixture over pastry in the pan. Smooth the surface. Top with the lamb mixture. Fold overhanging pastry over top.
Place 1 of the remaining pastry sheets on a clean surface. Top with another pastry sheet. Brush with butter. Continue layering with remaining pastry and butter. Place over the lamb mixture and tuck the edges into the pan. Sprinkle with caraway seeds and sesame seeds. Bake for 30 mins or until golden brown and heated through.
Combine the lemon juice, remaining dill, remaining mint and the remaining oil in a bowl. Serve pie with dill mixture.