Cook the potatoes in a large pan of boiling water for 15 mins until tender. Drain, return to the pan, add the butter, milk and season, then mash until smooth. Keep warm until you’re ready to serve.
Meanwhile, combine the mince, parmesan, grated apple and some seasoning. Shape the mixture into ping-pong-ball-sized meatballs. Heat half the oil in a wide pan over a medium heat. Fry the meatballs for a few minutes, stirring, until golden all over. Transfer to a plate.
Heat the remaining oil in the pan, and fry the onion for 8-10 mins until soft and translucent. Add the garlic, mushrooms and thyme, then season and fry for 10 mins more until the mushrooms are soft and most of the liquid has evaporated. Stir in the crème fraîche.
Cook the peas or green beans in a pan of boiling water for a few minutes until tender. Drain and leave to steam-dry. Stir the meatballs through the sauce and cook for 1-2 mins until hot and cooked through, then season. If you want to make the meatballs ahead, leave to cool. Will keep chilled for up to a day. Reheat thoroughly. Serve alongside the mash and veg.