1. Boil the swede for 20-25 minutes.
2. Meanwhile, heat a little oil in a pan and cook the onion, garlic, celery and mushrooms for ten minutes with the lid on, until soft but not browned. 
3. Stir in the tomato puree, tomatoes, lentils and stock and bring to the boil. Simmer for 20 minutes. 
4. Stir in the worcestershire sauce. 
5. Drain the swede and mash with the milk.
6. Preheat the grill on high. 
7. Put the mixture into an ovenproof dish and top with the swede mash. Add the cheese and grill to crisp up. 

These can be frozen for up to three months.