Heat oil in a large frying pan over medium-high heat. Add onion and garlic and cook, stirring, for 3-4 minutes until softened. Stir in beef mince and cook, breaking up mince with a wooden spoon, for 4-5 minutes until browned

Add carrot, celery and mushrooms and cook, stirring for 3 minutes. Add tomatoes, stock and Worcestershire sauce. Stir to combine, cover and bring to the boil. Reduce heat and simmer, uncovered, for 15-20 minutes until pan juices are reduced. Season with salt and pepper

Meanwhile, preheat oven to 200˚C. Cook sweet potatoes in a large saucepan of boiling water over high heat for 15 minutes or until tender. Drain and return to saucepan. Cook over low heat for a minute (to remove excess moisture). Remove from heat. Add butter and milk and mash until smooth

Spoon beef mixture into a 6-cup capacity ovenproof dish. Top with mashed sweet potatoes and swirl with the back of a spoon. Sprinkle with cheese. Bake for 20-25 minutes until hot and bubbling and serve