In a bowl, mix together the brown sugar, mirin, 1 tablespoon of the soy sauce and 2 teaspoons of the rice vinegar to make the marinade.
Place the salmon fillets in a shallow dish and spoon over the marinade. Leave to marinate for about 30 minutes.
Meanwhile make the salad, peel the cucumber and cut it in half across. Use a vegetable peeler to slice the cucumber lengthways into long thin ribbons; stop when you get to the seeds in the centre (discard these).
Place the cucumber ribbons in a large salad bowl along with the radishes, spring onions and watercress. Make the dressing by whisking together the remaining light soy sauce, sesame oil and remaining rice vinegar. Drizzle over the salad and toss gently.
Heat the barbecue to a medium-high temperature (or use a griddle pan if not barbecuing).
Barbecue the salmon, skin-side down, for 5 minutes then flip over and cook for a further 3-4 minutes, or until it’s opaque and the flesh flakes easily. Serve with the cucumber ribbon salad.
Cook's tip: You could also use this simple oriental marinade on chicken or prawn skewers, which are perfect for the barbecue as well.