Leave the clams in water with a little salt until ready to use. Scrub the shells very thoroughly.
Heat a wok over a high heat until smoke rises. Add the oil and swirl it around. Add the garlic, ginger and white spring onion. Stir and let them sizzle for a few moments to release their aroma. Add the mashed black beans and stir to mix. Tip in the clams. Sliding the wok scoop or metal spatula to the bottom of the wok, turn and toss for 30-45 seconds. Splash in the wine and sherry around the side of the wok, continuing to turn and stir. When the sizzling dies down, add the soy sauce, sugar and stock or water. Bring to the boil, cover, lower the heat to medium and cook for about eight minutes.
Remove the opened clams with a pair of chopsticks or tongs to a warm serving platter and keep warm. Stir and turn the remainder a few times and cook, covered, for another four to five minutes so that they will open. Transfer the rest to the serving platter, leaving the sauce in the wok. Discard any clams that do not open.
Lower the heat, add the well-stirred cornstarch to the sauce, stirring as it thickens. Tip in the green spring onion. Scoop the sauce on to the clams and serve immediately. Sesame oil may be sprinkled on, if desired.