Fill the kettle and boil it. Pop the haddock in a deep frying pan and pour in all the milk. Pop on a lid. Bring to the boil, then turn the heat down and simmer for 5 mins. Take off the heat. Lift the fish out of the pan. Put to one side. Keep the milk.
While the fish cooks, peel and finely chop the onion. Rinse and finely slice the celery. Peel and grate or crush the garlic. Peel the carrot. Finely dice it.
Warm a splash of olive oil in a pan. Add the onion, celery and carrot. Season. Fry for 5 mins over a medium heat till soft and glossy. Stir often. Stir in the garlic. Fry for 1 more min till the pan smells sweet.
Crumble the stock cube in a jug. Stir in 150ml of the milk from the fish pan. Top up with hot water from the kettle to make 650ml.
Stir the risotto rice into the pan. Add a splash of the stock from the jug. Cook and stir till all the stock is absorbed. Add another splash and carry on till almost all the stock has been absorbed and the rice is creamy and tender.
Flake the haddock into large chunks. Add to the pan. Rinse the spinach. Add to the pan as well. Cook and stir for 2-3 mins till the spinach is wilted. Taste and adjust the seasoning. Pop on a lid. Take the pan off the heat.
Separate the lettuce leaves. Rinse them. Dry in a salad spinner or pat dry with kitchen paper. Tear into smaller pieces. Pop in a bowl with a little olive oil and some seasoning.
Divide the risotto between two warm serving bowls. Serve with the dressed lettuce on the side.