Heat 2 tablespoons oil in a frypan over high heat. Cook lamb, breaking up any lumps with a wooden spoon, for 6-8 minutes until browned. Remove lamb from the pan and set aside.
Reduce heat to medium. Place onion in the frypan and cook, stirring, for 1-2 minutes until softened. Add garlic, ginger and curry paste, then cook, stirring, for 1-2 minutes until fragrant. Return lamb to the pan with tomato and kasundi, then bring to a simmer and reduce heat to low. Cook, stirring occasionally, for 15 minutes or until thick. Stir in peas. Transfer mixture to a 2L baking dish, level the surface and allow to cool.
Preheat oven to 200°C. Place potato in a saucepan of cold, salted water, bring to the boil and cook over medium-high heat for 12-14 minutes until tender. Drain, then mash until smooth. Add the coriander, turmeric, chilli, lemon juice and butter, then season and beat with a wooden spoon until smooth. Spread over lamb mixture, then bake for 20-25 minutes until golden.
Heat remaining ¼ cup (60ml) oil in a frypan over high heat. Fry curry leaves for 20 seconds or until crisp. Drain.
Serve pie with curry leaves and kasundi.
