Bring to boil over a high heat then put on cover and transfer to the pre-heated oven. Cook for 2 hours. Remove from oven and taste, then season. Add the mushrooms, stir through and then cook for another 30 mins. If you aren’t ready to serve straight away turn the oven off and leave the casserole in the oven until you are ready. When serving sprinkle some parsley and lemon zest on each plate.