Place the potatoes in a saucepan of salted water and boil until tender. Drain, add the butter and milk, then mash until smooth. Season with salt and set aside.
Heat the oil in a frying pan over medium-high heat. Add the beef mince and cook, breaking it up as it browns.
Add the onion and garlic and cook for another 2–3 minutes, or until softened.
Stir through the flour and cook for 1 minute.
Crumble in the beef stock cube, then add the water, salt and tomato sauce. Stir well and simmer for 5–7 minutes, or until the mixture has thickened.
Add the frozen mixed vegetables and cook for another 2–3 minutes.
Spoon the beef mixture into an ovenproof dish and spread evenly. Spoon the mashed potato over the top and gently spread it to cover the mince. Sprinkle with the grated cheese.
Bake at 180°C for 20–25 minutes, or until heated through and golden on top.