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Tip the lentils into a pan with the onion, carrot and celery. Pour in the stock and bring to the boil. Give it a stir, then reduce the heat, cover and simmer for 20-25 mins, until the lentils are tender.
Mix together the crème fraîche, half the dill and the lemon zest, adding a little seasoning. Put the fish in a shallow dish with a splash of water and cover with cling film. Microwave on Medium for 4-6 mins until the fish flakes easily.
When the lentils are cooked, stir in the spinach until the leaves are barely wilted, then stir in the crème fraîche mixture. Divide between 2 warmed plates and top with the haddock. Scatter over the remaining dill and serve.