Season the sea bass fillet and place, skin side down, in a hot ovenproof pan. Transfer to a hot oven and cook for three minutes.
Meanwhile, make the pesto. Toast the walnuts in a dry pan.
Put all the ingredients, except the olive oil, into a pestle and mortar. Pound thoroughly until you have a chunky paste.
Gradually add the olive oil to give the sauce a drizzling consistency.
Season to taste, and serve the sea bass fillet with the sauce.