Heat the oil in a large pan. Tip in the onion, and fry gently for 5 minutes until it's softened.
Stir in the rice and mix until it's all coated in the oil and onion.
Pour in the wine and let it bubble until it's almost disappeared.
Pour in a ladleful of hot stock. Give it a good stir, then leave it to gently cook until all the stock is absorbed. Continue adding the stock this way, stirring well and allowing the stock to be absorbed after each addition before adding more, until the rice is tender.
Stir the mackerel into the rice with the spring onions and spinach. Cook until the spinach has wilted slightly. Serve garnished with chives and a grind of black pepper.