It used to be that you’d have to soak clams in salted water with a little cornmeal, which would make them spit out any sand they had ingested. I no longer bother with this because I can’t remember the last time I ate a gritty clam, but if you don’t know where the clams came from, or if you harvested them yourself in the wild, it’s wise to let them purge.
Rinse the clams under cool running water, then drain in a colander. Thinly slice the garlic. Cut the bird’s-eye chillies in half lengthwise and scrape out and discard the seeds. Slice the chilli flesh about 3mm (⅛in) thick. Slice the banana chillies on the diagonal into 5mm (¼in) pieces. Pour the fish sauce, oyster sauce and sugar into a bowl and stir thoroughly.
Heat 20ml (4tsp) of cooking oil in a wok placed over a high flame. When the oil is hot, add the garlic and both types of chillies. Stir fry for about 30 seconds, then add the clams. Add the fish sauce mixture, stir well and bring to the boil. Lower the heat, cover the wok with the lid and simmer for a few minutes, stirring often. Cook until most of the clams open, then taste the sauce and correct the seasonings, as necessary. Add some black pepper and the Thai sweet basil leaves. Stir until the basil wilts, then serve immediately with steamed rice.