To make the saikyo miso, put the sake, mirin, miso paste and sugar in a medium saucepan and bring to the boil over a high heat. Cook for a few minutes, until the sugar has dissolved. Leave to cool to room temperature.
Put the cod in a bowl and cover with the saikyo miso (reserving a little to garnish). Cover and leave to marinate at room temperature for 2–3 hours.
When you’re ready to cook the cod, preheat the oven to 220C/200C Fan/Gas 7. Heat an ovenproof frying pan over a medium heat. Lift the fish fillets out of the marinade and cook them skin side-down in the pan for 2 minutes. Turn the fish over and roast in the oven for 6–8 minutes, or until cooked through and blackened.
Meanwhile, for the sesame spinach, bring the dashi stock to the boil in a large saucepan and add the soy sauce and sugar. Once the sugar has dissolved, add the spinach, stir and turn off the heat. Toast the sesame seeds in a dry frying pan until golden, then crush using a mortar and pestle. Drain the spinach, reserving the liquid, and squeeze out any excess moisture. Mix the spinach with almost all the toasted seeds and a little of the cooking liquor.
To serve, gather the spinach into a ball and top with the remaining sesame seeds. Serve with the cod and hajikami, with a few drops of the reserved saikyo miso on each plate.