Heat the butter and oil in an overproof dish on the hob.
Slice the leeks and cook until soft.
Add the arborio rice and cook for 2 minutes.
Add the milk and fish stock, bring to the boil then simmer for 5 minutes.
Cut the haddock into chunks and place on top.
Cover the dish and cook in the oven at 180 (fan) for 20 minutes.
Remove from the oven, stir in the creme fraiche and the spinach until the spinach has wilted.
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