In a large saucepan, heat oil over medium heat. Add onions, mushrooms, courgette and garlic. Cook and stir until tender, about 5 minutes.
Add lentils and 750ml water to vegetables. Bring to a rolling boil, stirring occasionally. Reduce heat to low, cover, and cook 45 to 60 minutes.
Stir in passata, tomato purée, sugar and remaining water. Bring to the boil. Reduce heat, cover, and simmer for 20 minutes. If necessary, add more water to keep the sauce from sticking. Be careful not to dilute; the sauce should be quite thick.