Place the skate wing in a dish large enough to sit in your large wok on a steamer stand, with enough space to cover the wok completely with a lid. Finely slice the ginger and place on top of the skate. Slice the spring onion lengthways into thin strips and place in a small bowl. Give the black beans a quick rinse under cold water, then tip them into another small bowl. Finely chop the garlic and add to the black beans with the salt. Lightly crush the ingredients together with the back of a teaspoon, then spoon the mixture over the top of the skate, spreading it across the length of the wing. Set your large wok or steaming pan up with a steamer stand and fill with boiling water to a third of the way up the sides. Place the fish dish into the wok or pan, cover with a lid and steam for 7–15 minutes, until the fish is cooked and the flesh is falling off the wing when tested with a fork. Remove the fish from the pan and scatter over the spring onion. Heat the vegetable oil in a frying pan until smoking-hot, then carefully pour the hot oil over the top of the fish. Spoon over the soy sauce and serve.