1.To make green sauce, combine all ingredients in a small bowl.
2.Heat oil in a medium saucepan; cook garlic and onion, stirring, until onion softens. Add beans and stock; bring to the boil. Reduce heat; simmer, uncovered, until liquid has almost evaporated. Stir in cream; blend or process bean mixture until smooth. Season to taste.
3.Meanwhile, cook fish, skin-side down, in a heated oiled large frying pan, turning once, until cooked as desired.
4.Serve fish on white bean purée, topped with green sauce.
Many varieties of cooked white beans are available canned, among them cannellini (which is what we used), butter and haricot beans; any of these are suitable for this recipe. We used kingfish in this recipe but you can use snapper or any white fish fillets you like.