BBC Food - Recipes - Flash-fried salmon with celeriac cakes

Ingredients

• 400g/14oz celeriac , grated
• 1 onion , finely sliced
• 1 free-range egg
• 2 tbsp plain flour
• salt and freshly ground black pepper
• 50g/2oz unsalted butter
• 2 tbsp olive oil
• 1 tbsp Dijon mustard
• 1 tbsp caster sugar
• 1 tbsp chopped fresh dill
• 1 tbsp cider vinegar
• 1 clove garlic , crushed
• 4 x 150g/5oz salmon fillets
• watercress
• 4 lemon wedges