For the celeriac: Peel celeriac and use a mandolin to slice it 2mm thin. Place slices into a cryovac bag, stacked no more than 3 high, with a little salt and olive oil. Steam these at 80°C for 15mins, then plunge into ice water.
For the slippery jacks: Remove all dirt from the top mushroom caps and then wash clean. Wipe with a clean cloth and allow to dry, cap side down, on a clean cloth in the fridge overnight. Slice 1cm thick. In a hot pan, in batches, fry the mushrooms in buerre noisette on one side to get a little bit of colour, finish the last 5 seconds with chopped thyme leaves. Do this on a high heat, and don’t crowd the pan.
For the blood pudding: Sweat onion and garlic, add spices and cook for 1 minute. Add blood and cook, stirring constantly for 6 minute over low heat. Add in chopped oats, and cook for a further 6 minutes, stirring constantly, add the soft butter and mustard, and remove from heat. Stir through until smooth, and taste for seasoning. Add salt and pepper to taste.
To assemble: Line a 50cm tray with baking paper. line the tray with ¼ of the celeriac slices, overlapping each slice to create a uniform pattern. Then spread out evenly 1/3 of the blood pudding mixture, and then spread 1/3 of the slippery jacks over blood pudding mixture. Repeat this process, finishing with the last ¼ of celeriac slices. Rest this lasagne in the fridge for 45mins. Steam at 100°C for 35mins. Make sure the centre is firm to touch. At this stage, you can chill or serve warm. You will get much cleaner portion if you can cool this down and then reheat to serve.
To serve: While the lasagna is cold, cut into desired number of portions. Steam these portions to reheat, or bake them with Parmesan to get a crust onto them. Serve with apple chutney, cos salad, or perhaps a roast pork rack.