Place the beetroot, fennel seeds, half a bunch of chervil, salt and sugar in a blender and puree until smooth. Place the salmon in a large bowl with the skin side down and spread the puree on top. Cover with cling film and marinate in the fridge for 10 hours.
After 10 hours, turn the salmon over and make sure the marinade is well distributed over the salmon. Leave to marinate for a further 20 hours. Remove the marinade from the salmon by rinsing with water and patting dry with kitchen paper. Cut into thin slices.
Serve with crusty bread, cream cheese with a little horseradish, cucumber and a little chervil. The salmon will keep in the fridge for at least a week.
Notes
For calculating the nutritional value I did not count the salt, sugar and beetroot as most of that is washed away.