Bonito flakes
Paper-thin shavings of dried, smoked and fermented bonito fish, smoky and deeply savoury.
Bonito flakes, katsuobushi in Japanese, are made from bonito fillets that are simmered, smoked and dried over weeks and often fermented with a mould until they are as hard as wood; fine shavings are then planed off. They taste smoky, fishy and intensely savoury, and together with kombu they form the basis of the broth dashi. The fine flakes curl on hot food and appear to dance, while thicker shavings are better for simmering into stock. They are sold in Asian grocery shops and by mail order, usually in small bags; once opened they should be kept airtight and cool, as they lose aroma quickly.
Substitute:
There is no true substitute, since the combination of smoke and fish savouriness is unique. For dashi, kombu alone or with dried shiitake makes a vegetarian broth that lacks the smoky, fishy note; as a topping on dishes, bonito flakes cannot be meaningfully replaced.