Preheat oven to 200°C.
Heat olive oil in a large saucepan over medium heat. Add onion and garlic and cook, stirring for 2 minutes or until onions have softened but not browned.
Add mince and cook, stirring until browned and broken up, about 7 minutes.
Add carrot, mushrooms and celery and cook, stirring for a further 2 minutes. Then add remaining ingredients and stir to combine. Bring to the boil, cover and reduce heat to medium-low. Simmer gently for 20 minutes, then stir through frozen peas.
Pour savoury mince into a large baking dish. Arrange potato gems on top then bake in preheated oven for 20 minutes.
Remove from oven, sprinkle with grated cheese and return to the oven for a further 20 minutes or until cheese has melted and gems are crunchy. Sprinkle with parsley and serve immediately.