Bring the stock to a simmer in a deep frying pan and lay the fish in it. Cover and poach for about 4 mins, or until the fish begins to flake. Remove from the heat, then strain the cooking liquor into a jug, reserving it for later. Leave the fish in the pan, covered.
Melt the butter in a large saucepan over a low heat and add the leeks. Season, then add about 2 tbsp of water, cover the pan and cook the leeks for about 10 mins until softened. Stir in the barley, turning it over in the juices for 1 min, then increase the heat a little and add the vermouth. Let this bubble away to almost nothing, then add half the reserved fish stock.
Cook the barley in the stock, over a medium heat, while stirring from time to time, for about 15 mins. (The stirring helps the barley become creamier.) Add the rest of the stock and cook for another 20 mins, stirring from time to time, adding more boiling water if the mixture is getting too dry. It might take a little longer to cook the barley – the grains should be soft but with a little bite in the centre.
Remove the skin from the fish – if there is any – and break the flesh into big flakes. Add this to the risotto along with the spinach and parsley, stirring carefully as you don’t want to break up the pieces. Taste for seasoning. Stir in the cream, if using, and check again for seasoning. Serve with the cheddar grated on top, if you like.