Preheat the oven to 200°C, fan 180°C, gas 6. In an ovenproof casserole, gently fry the leeks for 5-10 minutes until soft and just golden. Add the garlic and rice, and cook for a further 2 minutes.
Pour in the stock and milk and bring to the boil, then simmer for 10 minutes. Remove from the heat and place the haddock on top of the risotto. Cover the dish with foil, then bake in the oven for 15 minutes, until the rice is tender and the haddock is cooked through.
Stir in the crème fraîche, lemon juice, spinach and peas, breaking up the haddock into small pieces. Leave to sit for 2 minutes until the spinach has wilted slightly. Season and sprinkle with the parmesan to serve.