Preheat oven to 180°C. Sauté the onion, celery, carrot and garlic in extra virgin olive oil for 5 minutes in a medium saucepan, then set aside. Fry the mince in the same saucepan until browned, then return the onion mixture, pour into the tomato paste, stock and Italian herbs, then simmer for 45 minutes. Season with salt and pepper.
Steam the sweet potatoes until tender, then purée until smooth. Season with salt and fold in the Parmesan. Spoon the beef filling into individual baking dishes, top with the sweet potato purée, then bake for 10 minutes.