Preheat the oven to 90C (non-fan)/Gas ¼. Lay the herbs in a single layer on a baking sheet and cook for 1 hour or until dry. Once dried, blend to a fine powder, and place in a flour duster.
To cook the shellfish, wash the mussels and remove any beards. Wash the cockles. Discard any open shellfish that do not close when given a sharp tap.
In a hot pan add the olive oil and cook the garlic for 1 minute. Add the rest of the ingredients and the cockles and mussels. Cover with a lid.
Cook for 2 minutes, or until the cockles and mussels have opened. Discard any that do not open.
Pour through a colander lined with a muslin cloth into a clean pan. Keep the liquid to make the sauce. Pick out the shellfish. Chop 10g of the largest mussels for the mussel butter. Set aside the remaining shellfish.
For the mussel butter, mix all the ingredients in a bowl with the 10g of chopped mussels.
For the mussel sauce, in a saucepan, heat the shellfish cooking liquid until reduced to 50ml. Add the crème fraiche and lime juice. Add almost all the mussel butter, keeping back a tablespoon for the cabbage. Blend the sauce with a stick blender. Stir in the chives and chervil.
For the cabbage, preheat the oven to 200C/180C Fan/Gas 6. Cut the cabbage into 8 wedges. Heat the chicken fat in an ovenproof, non-stick frying pan until hot. Fry the cabbage until coloured. Roast in the oven for 5–7 minutes.
Remove from the oven and baste with a spoonful of mussel butter, until golden. Season with lemon juice and salt.
To serve, place the cabbage in the middle of the plate. Reheat the shellfish in a little sauce, and spoon them over the cabbage. Pour over the remaining sauce and dust with the green powder.