Fillet the mackerel and remove the first layer of skin. Take out any pin bones using tweezers. Reserve the bones.
Dissolve the salt in the water and add the mackerel to the solution. Leave for 4 hours.
Place the mackerel bones on a cake rack and put the hay underneath in a deep heat- proof tray. Set fire to the hay and allow it to burn, being very careful of the flames, before covering the bones with tin foil to smoke them.
Place the bones in 2 litres of filtered water and bring to a simmer over a gentle heat. Drop the kombu into the simmering water for 2 minutes, then remove. Allow the broth to gently simmer for 2 hours, then pass through a muslin cloth to strain.
After the mackerel has been in the brine for 4 hours, remove and wash.
Ladle a small amount of the smoked mackerel stock into each warm bowl.
Burn the top of the mackerel with a hot ember from a barbecue or a blowtorch, being careful not to cook the mackerel but warm it. Slice and add to the bowls.
Grate the horseradish and arrange on the top of the fish. Serve immediately.
Recipe from wilsonsbristol.co.uk