Cut chicken into 15mm chunks and sauté shallots, garlic, chilli, and ginger in oil until soft.
Stir in curry paste and chicken, then add coconut cream, stock, lime leaves, lemongrass, and sweet potato.
Simmer for 15 minutes until potato is tender, then stir in cornflour slurry to thicken.
Season with fish sauce and pepper, cool the mixture, remove herbs, and transfer to a 1.2 litre dish.
Rub butter and lard into flour and salt until the mixture resembles fine breadcrumbs.
Mix in two tablespoons of water and lemon juice until the dough comes together.
Knead into a ball, wrap in film, and chill in the fridge for 30 minutes.
Preheat the oven to 200°C.
Roll out pastry, create a lattice top, place over filled dish, seal edges, and brush with egg.
Bake for 30 minutes until golden and rest for five minutes before serving.