Preheat oven to 200°C/180°C fan-forced. Place cornflour in a jug. Gradually add 1/4 cup stock, stirring until smooth. Add curry powder, garlic and remaining stock. Stir until combined.
Place sausage and onion in a 20 x 25cm (4cm deep) baking dish. Pour over stock mixture. Bake for 40 minutes or until sausage is golden. Scatter over peas.
Heat mash according to packet instructions. Dollop mash over sausage mixture and use back of a spoon to swirl. Sprinkle with cheese. Bake for a further 10 minutes or until cheese is golden. Season with pepper and serve.