1. Preheat the oven to 180°C/fan160°C/gas 4. Cook the onions in 1 tbsp oil in a casserole until soft. Turn up the heat, add the mince and cook for about 10 minutes, until browned.
3. Meanwhile, boil the potatoes for 10-12 minutes, until tender. Drain, add the milk, butter, seasoning and the remaining olive oil and mash.
4. Take the mince out of the oven and turn the temperature up to 220°C/fan200°C/gas 7. Tip the mince into a 1.3 litre ovenproof dish. Spoon the potato on top to cover the mince. Put the dish on a baking tray and cook for 20 minutes, until the top is golden.