Heat oil in a large, non-stick frying pan. Add onion. Cook, stirring, for about 3 minutes until softened.
Stir in lentils, passata, stock cube, herbs and 500 ml hot water. Bring to the boil. Reduce heat, simmer for 15-20 minutes, until lentils are tender, stirring occasionally. Stir in frozen vegetables. Simmer for a further 5 minutes. Transfer to an ovenproof dish (8-cup capacity).
Meanwhile to make topping, cook sweet potato and potato in a large saucepan of boiling water for 20 minutes or until tender. Drain and return vegetables to pan. Add milk, season with pepper and mash over low heat until smooth.
Spoon mash over lentil mixture in dish and rough up surface with a fork or the back of a spoon. Sprinkle with cheese. Bake in preheated 200 °C oven (fan-forced) for 20-25 minutes, or until topping is lightly browned.