Chestnut mushrooms with dashi, sour onions and gooseberry 'boshi'

Begin by making the sour onions and gooseberry boshi, as both these need at least 4 weeks to mature. You’ll be creating more than you need for this recipe, but they keep indefinitely and are an amazing ingredient to have in the pantry

For the sour onions, remove the roots and slice the white parts as finely as possible. Weigh your sliced onions, then work out 2% of this weight and add that amount of salt (for example, 500g of sliced onions would require 10g of salt). Scrunch the salt and onions together, then set aside

Place the rest of the ingredients for the sour onions in a saucepan and bring to the boil, then remove from the heat and chill. Place the salted onions in a vacuum bag and add the pickle liquid, then compress for 35 seconds. If you don’t have a vacuum packer, you can also place the onions in a sterilised jar and pour the pickling liquid over them – but you must make sure they remain fully submerged. Leave to pickle at room temperature for 4 weeks, away from direct sunlight. If pickling in the jar, you may want to open the lid occasionally to release any built-up gas

For the gooseberries, there are no set measurements – just get hold of the best gooseberries you can. Neil gets his from Brambletye Fruit Farm, which is where he’s found the best red gooseberries. Sprinkle a layer of rock salt in the base of a Kilner jar, then add a layer of cleaned gooseberries. Top with a few shiso leaves, then repeat these layers until the jar is full (or you run out of gooseberries). Leave for 4 weeks at room temperature until ready to use – the gooseberries will become submerged in liquid and the salt may sink to the bottom, but this is fine. These gooseberries will last indefinitely and continue developing flavour. Before using them, just rinse the gooseberries under water

The day before you plan to serve the dish, prepare the dashi. Combine all the ingredients in a saucepan and bring to a rolling boil. Remove from the heat, add a pinch of salt and set aside to infuse overnight in the fridge

The next day, preheat an oven to 170°C/gas mark 3½ and light a barbecue to cook the mushrooms. You could also use a very hot griddle pan, but they won’t have quite the same smoky flavour. Wash the mushrooms if needed, then remove the centre stalks (these can be kept for fermenting if desired). Brush the flat side of each mushroom with oil and place flat-side down on the hot grill. Cook until the underside is blackened (note you don’t want to cook the mushroom completely, just char the flat side)

Once charred, remove the mushrooms from the grill and place them into 1 or 2 roasting dishes, ideally so they snugly fit in a single layer. Strain the dashi stock and pour enough into the dish(es) until the mushrooms are half submerged. Add the rapeseed oil, basil and soy sauce, then roast for 20-30 minutes, basting the mushrooms after the first 10 minutes. Any leftover dashi can be frozen and used in other dishes

While the mushrooms cook, bring a small pan of oil to 180°C. Deep-fry the spring onion roots (or celeriac roots or finely sliced leek, depending on what you’re using) for a few minutes until crisp, then drain on kitchen paper. Toss the crispy roots in the salt, nori, porcini and onion powder and set aside to garnish

To make the herb oil, simply mix all the ingredients together in a small bowl and add a pinch of salt

To serve, arrange a thin but wide layer of the sour onions on the base of each dish. Arrange the roasted mushrooms in a circle, flat-side up, then top with a few of the salted gooseberries, sliced into small pieces. Taste the cooking liquid from the mushrooms and adjust the seasoning if necessary with more soy sauce and/or a dash of lemon juice, then pour this into each bowl to create a broth. Finish with dots of the herb oil, a few more sour onion slices and the crispy roots