Oven: Preheat the oven to 200℃ (180℃ fan)/400F (350F fan).
Add the potato chunks to a large saucepan of salted water. Bring to the boil, then simmer until fork tender, about 15 to 20 minutes.
While the potatoes cook, heat a large frying pan over a medium heat. Add the onion and cook for 4 to 5 minutes until softened.
Stir in the garlic and cook for 1 minute.
Add the lamb mince and cook until browned, breaking it up as it cooks.
Stir in the tomato puree and tomato ketchup.
Add the salt, pepper, Worcestershire sauce, crumbled stock cube, thyme, all purpose seasoning, and mixed herbs. Add a splash of water if needed to help it simmer.
Simmer for 5 to 7 minutes, then stir in the frozen mixed vegetables and cook for a further 3 to 5 minutes until hot through and the filling is saucy but not watery.
Tip the mince mixture into a baking dish and spread into an even layer.
Drain the potatoes well, then mash until smooth. Stir in the milk, butter, Greek yoghurt, salt, and pepper until creamy.
Spoon the mash over the mince layer and spread to cover. Rough up the surface with a fork.
Sprinkle the cheese evenly over the mash.
Bake for 25 to 30 minutes, or until the top is golden.
Rest for 5 minutes before serving and enjoy!