This is an another old favourite from ” Not just a Load of Old Lentils” by Rose Elliot. There are various versions of this dish on the internet( and indeed Rose herself later updated the recipe to what is probably a healthier version) but this remains my preferred version. I use ready made bechamel sauce mostly these days but you can make it from scratch if preferred- its not tricky, im just a lazy cook at times. I also make the croustade in a food processor rather than the old ” rubbing in ” method.  You can use any mushrooms but i prefer chestnut or portabello as they have a good flavour. You could also add in a few wild mushrooms if available at a good price.The dish can also be made with leeks or courgettes. I think this  tastes even better the next day if there are any leftovers.