Place the grated celeriac in a clean tea towel or cloth and squeeze out the juices.
In a bowl combine the celeriac with the onion, egg and flour. Season well with salt and freshly ground black pepper and divide the mixture into four. Shape each piece into a ball and flatten into a cake shape.
Melt the butter with one tablespoon of oil in a frying pan over a medium heat. Fry the celeriac cakes for about 3-5 minutes on each side. Remove from the pan with a slotted spoon and set aside to drain on kitchen paper.
For the dressing, whisk all the ingredients together in a bowl.
Cut the salmon fillets into three long strips. Rub one side of the fish strips with oil and fry in a very hot frying pan for about one minute on that side only.
To serve, place a celeriac cake onto each of four plates, top them with salmon, garnish with lemon and watercress and spoon the dill dressing over.