Serrano chiles
A small, slender green chile from Mexico, usually used fresh. Noticeably hotter than a jalapeño, with a bright, grassy flavour.
Fresh serranos are sold in Latin American shops, sometimes in Asian grocers or well-stocked supermarkets, and otherwise by mail order. They are thin-walled and can be finely chopped raw into salsas without peeling.
Substitute:
Jalapeños come closest in flavour but are milder and fleshier, so use a little more. Slim green Thai chiles match the heat better but are usually even hotter and less grassy, so use fewer.