In Asia, razor clams are known as bamboo clams, because they look something like the fast-growing plant. They have sweet, tender meat and are easy to prepare - detach the body from the shell, and, without cutting it in half, slit the body lengthwise down the centre from one end to another. Squeeze out and discard the green-grey gooey stuff, then pull out the intestine. Rinse the clam meat under cool, running water, then drain and pat dry with paper towels.
Fresh green peppercorns are sold at shops specialising in southeast Asian ingredients. If you can't find them, use about one tablespoon of dried green peppercorns - but not the type in brine.
Cut the clam meat into two-bite pieces. Chop the onion. Remove and discard the stem and seeds from the bell peppers, then cut the peppers into strips about 1cm (7/16in) wide. Finely mince the chilli.
Heat a wok over a medium flame and add about 120ml (½ cup) cooking oil. When the oil is hot, add the branches of green peppercorns. Fry them briefly - just enough to darken the peppercorns - then remove them from the oil and drain on paper towels. Set aside one or two branches for the garnish, then pull the remaining peppercorns from the stems. (If using dried green peppercorns, fry them briefly until they darken slightly, then drain on paper towels.)
Pour off all but about 30ml (2tbsp) of oil from the wok, then heat it over a high flame. When the oil is hot, add half the bell peppers and fry them quickly so they char slightly but don't become too soft. Remove them from the wok and fry the remaining bell peppers the same way.
Add 30ml (2tbsp) of oil to the wok and place it over a medium flame. Add the onion and stir-fry until it starts to soften. Add the razor clams, de-stemmed green peppercorns (fresh or dry), chillies, fish sauce, sugar and salt. Stir-fry for about a minute, then add the bell peppers back to the wok. Stir-fry until the clams are cooked, then taste for seasoning and correct, if necessary. Stir in the butter and basil leaves before putting the ingredients on a plate. Garnish the dish with the reserved branches of fresh green peppercorns (if using), then serve.