Preheat the oven to 180 degrees celsius, on a fan forced setting.
Place the sweet potato in a pot covered in water and boil for 20-25 minutes until soft.
Heat the olive oil in a saucepan and sauté the onion and celery for 3-4 minutes.
Add the lamb mince to the pan and brown for 5 minutes.
Add the mushrooms, diced tomatoes, parsley, salt and pepper and cook for 20 minutes.
Transfer the lamb mixture to a baking dish.
Mash the boiled sweet potato and spread over the lamb mixture in the baking dish. Place in the oven to cook for 10 minutes.
Remove from the oven and serve.