Pour the milk into a saucepan large enough to take the fish. Place the fish in the milk, add the bay leaves and peppercorns, then bring to the boil.
As soon as the milk shows signs of foaming, lower the heat and simmer for 8-10 minutes or until the fish is just tender. Turn off the heat and leave the milk to infuse with the fish and aromatics.
Gently heat the fish or vegetable stock in a saucepan. Peel and finely chop the onion, then fry gently in the butter in a broad, heavy-bottomed pan. When the onion is soft and translucent, but before it colours, add the rice and briefly stir it through the butter to coat the grains.
Pour in the wine, let it almost entirely evaporate, then add the stock a ladle at a time, allowing each one to be soaked up by the rice before adding the next.
Once all the stock has gone, change to the milk, strained of its peppercorns and bay leaves. By the time almost all the liquid is absorbed the grains should be tender and plump yet with a slight bite to them.
Fold in a thin slice of butter and season carefully. The total cooking time will be about 20 minutes, maybe a few minutes longer.
Wash and tear the spinach leaves into small pieces then stir them into the rice. Break the fish into large, juicy flakes and add them to the rice, folding them in, but keeping the flakes as whole as possible. Check the seasoning and serve.