This is a popular dish at seafood restaurants in Hong Kong. It can be made with any of the larger varieties of clam, including bamboo clams (or razor clams) and baby geoduck.
Pre-minced garlic from supermarkets often has an unpleasant metallic taste, so don't use it for this dish, and also resist using a garlic press, because the taste is to strong. Instead, mince the garlic with a sharp knife.
Most of the clams we buy from supermarkets and seafood vendors have been purged of the grit that they ingest. If you harvest the clams yourself, or are worried that they might be gritty, soak the live clams in cold salted water with a bit of cornmeal or flour, and leave them for about an hour. Rinse the clams in fresh water, scrub the shells, then proceed with the recipe.
Soak the mung bean noodles in warm water for about 15 minutes, or until pliable. Drain them in a colander and squeeze out the excess liquid. Use kitchen scissors to cut the noodles into shorter lengths.
Remove and discard the top shell from the clam, then detach it from the bottom shell by cutting through the tough muscle. Lay the clams back in the bottom shell and top with the mung bean noodles.
Mince the garlic and cut the chilli (if using) into thin rounds, squeezing out and discarding the seeds as you go. Julienne the spring onion.
Scatter the minced garlic and chilli over the clams and lightly drizzle a little soy sauce over each one.
Place the clams in one layer on a large shallow plate and put it on the tier of a steamer, or on a low rack in a wok. Pour boiling water into the bottom of the steamer (or wok). Cover the steamer (or wok) with the lid and bring the water to the boil over a high flame. Turn the heat to medium-high and steam the clams for about five minutes, or until they're done.
While the clams are steaming, heat the oil in a small pan until very hot.
When the clams are cooked, put the spring onions on top and drizzle with the hot oil. Serve immediately with steamed rice and stir-fried green vegetables.