1. Preheat the oven to Gas Mark 6/200 ?C/400 F. 2. Grease a 28x23cm (11x9in) ovenproof dish. 3. To make the cannelloni filling, melt the butter in a saucepan, add the onion and fry for 5 minutes. 4. Add the mushrooms and cook for 3-5 minuets. 5. Squeeze as much water as possible out of the spinach with your hands. 6. Add the spinach and boil for about 5 minutes until most of the water has evaporated. 7. Remove from the heat. 8. Stir in the parmesan. 9. Taste and adjust the seasoning. 10. To make the sauce, melt the butter in a saucepan. 11. Stir in the flour and cook for 1 minute, stirring. 12. Remove the pan from the heat and gradually stir in the milk. 13. Bring slowing to the boil and continue to cook, stirring continuously, until the sauce has thickened. 14. Remove the pan from the heat and stir in three-quarters of the cheeses, stirring until melted. 15. Taste and adjust the seasoning. 16. Pour enough of the sauce into the ovenproof dish to just cover the bottom. 17. Spoon the spinach filling into the cannelloni tubes and arrange them side by side in the dish. 18. Pour over the remaining sauce. 19. Sprinkle with the remaining cheese and bake in the oven for 25-30 minutes. NOTES : An attractive dish, ideal for a light supper.