Preheat the oven to 180˚C, fan 170˚C, gas 4.
Peel and quarter 4 baking potatoes and boil in salted water until tender.
Meanwhile, heat 1 tablespoon olive oil in a pan and fry 2 onions, peeled and chopped, and 3 carrots, peeled and diced, until tender.
Add 500g beef mince and cook, stirring. Once cooked through, season with salt and pepper and add 250ml beef stock.
Drain and mash the potatoes, adding 1 tablespoon mustard and 100ml semi-skimmed milk. Place the mince mixture in an ovenproof dish, cover with the mash and cook in the oven for 20 minutes until the pie is piping hot throughout and the top is golden.
Meanwhile, steam 2 courgettes, sliced, and serve alongside the pie.