Rinse the black beans in several changes of cold water and drain in a small sieve. Crush the beans in a small bowl with the back of a spoon. Add the garlic, ginger, cornflour and soy sauce. Mix well to form a thick paste.
Make 3 slashes on either side of the fish and spread the paste evenly over both sides. Set aside for 30 minutes.
Steam the fish over low heat for 10 to 15 minutes until the flesh turns opaque and flakes off easily with a fork.
Remove the fish and garnish with spring onions and corinader sprigs. Serve immediately.