Make a 5% salt brine by whisking 50g salt into 1kg water in a mixing bowl, until the salt has dissolved completely. Peel the potatoes and shred them very finely into fries. Place the fries directly into the brine — there is no need to wash away their starch, but they will turn brown if left exposed to the air.
Thoroughly wash and rinse a large glass Kilner jar, as well as a jam jar small enough to fit inside it. Place the jars in a low oven for about 10 minutes to dry and sterilise them. Briefly boil the Kilner jar’s rubber seal in a little water to sterilise that as well. Allow the jars to cool properly.
Transfer the fries to the larger jar. Place the little jam jar on top of the fries as a weight to keep them submerged, and pour in enough of the brine to cover the fries by about 3cm, and to fill the smaller jar. Seal the 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 — fermentation is, of course, also a preservation method.
For the sauce, roast the hazelnuts in a 160C oven (180C non-fan) with a drop of vegetable oil for 15-20 minutes until golden brown. Meanwhile, whisk together the egg yolks, mustard and vinegar in a small bowl, then slowly stream in the rapeseed oil, whisking vigorously all the time until you have a thick yellow mayonnaise. Season with salt and set aside.
When the hazelnuts are ready, lightly crush them with the back of a frying pan, season them with salt and allow to cool on kitchen paper. Fold about half the nuts into the mayonnaise, along with the chervil, pickled onion and bean shoots, and season the mixture with a little pickled onion vinegar. Keep the remaining hazelnuts to garnish the plates.
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. You will then be able to slice each wing in half with a large knife to give you four portions. You will be able to see that each portion has a thicker fillet on top and a much smaller one on the bottom.
Drain the fries well and pat them dry on kitchen paper. Heat vegetable oil in a deep fat fryer, or to no more than halfway up a very deep saucepan, to 180C.
Heat a little 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 hot pan thick side down and fry for about 2 minutes, then add the diced butter and cook for a further minute until the butter starts to brown and smell delicious. Flip each portion over and cook the thin undersides for about a minute, while basting the top with the foaming brown butter. Allow the fish to rest on a warmed plate for about 2 minutes.
Fry the drained fries until crispy. There should be no need to season them any more.
To remove the fish from the bone, insert a small, sharp knife between the thicker fillet and the bone at the thick edge of each portion, and 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 portions. You could serve the skate on the bone, as it is easy to remove the fillets with a knife and fork.
Place a good dollop of the sauce in the centre of each of four warmed dinner plates, and sprinkle with some of the reserved crushed 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.