Squeeze the juices out of the grated celeriac using a clean cloth.
Combine celeriac, onion, egg, and flour in a bowl and season with salt and pepper.
Divide the mixture into four balls and flatten them into cake shapes.
Melt butter with one tablespoon of oil in a frying pan over medium heat.
Fry the cakes for 3 to 5 minutes on each side until golden.
Remove cakes with a slotted spoon and drain on kitchen paper.
Whisk the mustard, sugar, dill, vinegar, and garlic together for the dressing.
Cut the salmon fillets into three long strips each.
Oil one side of the salmon and fry in a very hot pan for one minute on that side only.
Serve the salmon on top of the celeriac cakes with watercress, lemon, and dressing.