First, prepare the marinade. Combine the dashi, vinegar, soy sauce, mirin, sake and sugar in a saucepan. Place over medium heat and cook until almost boiling, then remove from the heat and leave to cool. Add the lemon juice to the cooled marinade.
Season the salmon with salt and pepper and lightly coat with the cornflour.
Heat half the oil in a frying pan over medium heat and fr