Preheat oven to 400°C Heat the oil in a large frying pan over medium-high heat. Add the onion and garlic and cook, stirring, for 5 minutes or until onion softens.
Add the lamb and cook, stirring with a wooden spoon to break up any lumps, for 5 minutes or until the meat changes colour. Add the flour and cook, stirring for 2 minutes or until well combined.
Add the beef stock, tomato paste and Worcestershire sauce and cook, stirring occasionally, for 5 minutes or until sauce thickens slightly. Remove from heat and set aside to cool slightly. Add mint and stir to combine. Taste and season with salt and pepper.
Meanwhile, use a 3 inch round pastry cutter to cut discs from the pastry sheets. Line the base and sides of 48 (1 1/2 tbs capacity) mini muffin pans. Use a fork to lightly prick the bases of pastry cases. Place in the fridge for 15 minutes to rest. Bake in oven for 15 minutes or until golden brown and crisp.
Place the potatoes in a large saucepan and cover with cold water. Place over high heat and bring to the boil. Cook for 15 minutes or until tender. Drain well. Use a potato masher or fork to mash until smooth. Add the cream and stir to combine. Taste and season with salt and pepper.
Spoon the lamb mixture among the pastry cases. Spoon the mashed potato over the lamb mixture. Drizzle with the melted butter.
Bake in oven for 10 minutes or until heated through and golden brown. Place on a serving platter to serve.