Heat oil in a heavy-based saucepan over medium-high heat, add mince and cook, breaking up lumps with a wooden spoon, for 8 minutes or until liquid has evaporated. Add onion, carrot, celery and garlic and cook, stirring often, for a further 5 minutes. Sprinkle in flour and stir for 1 minute or until well combined. Stirring continuously, add beer, stock, Worcestershire sauce, mint and cumin, bring to the boil and cook, stirring often, for 20 minutes or until sauce has thickened and lamb is tender. Season and pour into an 8-cup (2-litre) baking dish.

In a saucepan, cover potato with cold salted water, bring to the boil and cook for 15-20 minutes until very tender. Drain in a colander. Add milk to saucepan, bring to a simmer, then take off heat, add potato and mash until smooth. Fold in yolks, nutmeg, two-thirds of the cheese and season.

Preheat oven to 200°C. Top lamb with mash, scatter with remaining cheese and bake for 30 minutes or until potato is golden and sauce bubbling. Cool slightly for 15 minutes before serving.