Tempeh
Fermented soybeans bound by a mould culture into a firm block, nutty and faintly mushroomy.
Tempeh is sold in health food and organic shops, in Asian grocers and now in the chilled section of many supermarkets, usually vacuum-packed. Where it is missing, it can be ordered online and freezes well.
Substitute:
Firm tofu, well pressed, comes closest in use, but it is softer, smoother and neutral in taste, without the nutty, fermented note and the grainy bite.