On our first evening in Bangkok, the street lights glowed yellow over the dusty tangle of electrical cables which garland every roadside. An elderly woman tended the charcoal burner perched on top of her ramshackle barrow, hardly bigger than a tea trolley, pushed to the edge of the pavement.

We paused, our group of seven, gathering shyly around our guide Gung, and watched while the woman heated a round dimpled pan, like a giant iron solitaire board, and deftly poured in white liquid from a metal teapot.

“Salted rice flour and coconut milk,” explained Gung, with the serious manner of a school prefect in charge of the newbies. Without breaking rhythm, the cook picked up a second teapot and began...