Preheat oven to 200°C/180°C fan-forced. Heat oil in a non-stick frying pan over medium-high heat. Cook mince in 2 batches, stirring with a wooden spoon to break up mince, for 8 to 10 minutes or until browned. Transfer to a bowl. Add onion and garlic to pan. Cook, stirring, for 3 minutes or until softened.
Return mince to pan. Add gravy powder. Cook, stirring, for 1 minute. Stir in tomato. Bring to the boil. Reduce heat to medium-low. Simmer, stirring occasionally, for 25 to 30 minutes or until thickened.
Meanwhile, cook potato in a saucepan of boiling, salted water for 10 to 12 minutes or until tender. Drain. Return to pan over low heat. Mash until smooth. Add butter and milk. Stir until butter has melted and mixture is combined.
Spoon mince mixture into a 1.4 litre casserole dish. Top with potato mixture. Sprinkle with cheese. Bake for 20 to 25 minutes or until golden. Serve.