To make the fries, thoroughly wash a very large, sealable glass jar, as well as a jam jar small enough to fit inside it. Carefully place the jars in a very low oven for about 10 minutes to sterilise them. Briefly boil the glass jar’s rubber seal to sterilise too. Leave the jars and seal to cool.
Make the salt brine by whisking the salt into 1 litre/1¾ pints of water in a mixing bowl, until the salt has dissolved completely. Place the potato fries into the large jar and pour in enough of the brine to cover the fries by about 3cm/1¼in. Fill the smaller jar with the remaining brine and place on top of the potatoes to enure they stay submerged in the brine. Seal the larger jar and leave it on a shelf (not in the fridge) for 3–5 days, returning each day to “burp” the jar by opening it to release any gases that have built up. The fermentation is complete when you open the jar and it no longer pops and fizzes. The jar can then be transferred to the fridge, where the fries will keep for several weeks.
For the sauce, preheat the oven to 180C/160C Fan/Gas 4. Roast the hazelnuts, drizzled with a splash of vegetable oil, for 15–20 minutes, or until golden brown.
Meanwhile, whisk together the egg yolks, mustard and vinegar in a small bowl. Add the rapeseed oil in a thin drizzle, pouring very slowly and whisking vigorously all the time to make sure each drop of oil is incorporated before adding more. When you have a thick yellow mayonnaise, season with salt and set aside.
Lightly crush the hazelnuts with the back of a frying pan, season them with salt and leave to cool on kitchen paper. Fold about half of the nuts into the mayonnaise, along with the chervil, pickled onion and beansprouts, and add the splash of pickling vinegar. Set the sauce aside.
For the fish, using a small, sharp knife, cut along the large bow-shaped bone on both sides of each wing to expose the fine bones in the centre between the fillets. Using kitchen scissors, cut through these bones and remove the large bone. Slice each wing in half with a large knife into four portions.
Drain the fries well and pat them dry on kitchen paper. Preheat a deep-fat fryer to 180C. (CAUTION: hot oil can be dangerous. Do not leave unattended), Cook the fries in the deep-fat fryer until golden and crispy. Carefully transfer the fries to kitchen paper to drain.
For the skate, heat the vegetable oil in a large heavy-bottomed frying pan over a medium–high heat. Pat the skate portions dry with kitchen paper and season generously with salt. Place the fish in the pan, thick-side down, and fry for about 2 minutes. Add the butter and cook for a further minute until the butter starts to brown. Flip each portion over and cook the thin undersides for about a minute, while basting the top with the foaming brown butter. Leave the fish to rest on a warmed plate for about 2 minutes.
Once cool enough to handle, remove the fish from the bone by inserting a small, sharp knife between the thicker fillet and the bone at the thick edge of each portion. Slice along the bones to the thin edge to remove the fillet. Repeat this on the other side of each portion to free the thin fillet beneath. Sandwich the now boneless fillets back together in four portions.
Add the tartare sauce in the centre of four warmed dinner plates and sprinkle over some of the reserved hazelnuts. Place the fish portions over the sauce, then top with more crushed nuts. Cover each portion of fish with a large helping of freshly fried fermented fries and serve immediately.