Preheat the oven to 180 degrees Celsius.
Roast the pine nuts in a dry pan until golden and set aside.
Remove and discard the thicker stalks from the watercress, then puree in a blender, adding the pine nuts and preserved lemon and finally pouring the olive oil into the mix.
Make sure the fish is completely clean, and wipe over with a paper towel or a dry tea towel. Score the skin of the fish and smear the watercress paste all over the fish, inside and out.
Wrap the fish in foil, place on a baking tray and bake in the oven for 30 minutes before removing the foil and baking for a further 15 minutes.
This tastes great served with grilled mushrooms drizzled with a dressing of shoyu and lemon juice, and some steamed Asian greens.