Stir together the miso, mirin, sugar and sake, if using. Place the salmon fillets on a plate or in a medium food bag and cover with the miso marinade. Leave in the fridge for at least 30 mins or, better still, overnight.
Heat oven to 200C/fan 180C/gas 6. Lightly oil a baking tray. Use kitchen paper to wipe off the marinade (don’t be tempted to wash it off as you will lose some of the flavour). Place the fillets on the baking tray, skin side down. Cook in the oven for 15 mins until the fish flakes easily and is cooked through.
Meanwhile, make the cucumber salad. Using a vegetable peeler, shave the cucumber into long, thin ribbons. Whisk together the rice wine vinegar, oil and sugar. Toss the cucumber ribbons in the dressing, then curl them up on two plates. Serve with the salmon and some plain white rice sprinkled with toasted sesame seeds.