Heat oven to 200C/fan 180C/gas 6. Spread the sesame seeds over a large plate. Season the salmon fillets if you like, then dip into the beaten egg and roll in the sesame seeds. Place on a baking tray and roast for 10 mins or until firm and the sesame seeds are golden.
Split sub rolls, fill with a little of the spinach and watercress salad, then top with the salmon fillet. Serve with the cucumber pickle on the side and soy sauce for splashing, if you like.