You can also purchase finely shredded kombu, available online at sources such as theingredientfinder.com.

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It is traditionally made with water, kombu, a dried seaweed, and dried, smoked bonito fish flakes.

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Now he goes after sea urchins and collects kombu seaweed, though wholesalers won't pay much for either.

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Kombu is available at most Asian markets as well as some of the larger American supermarkets, such as Whole Foods.

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This form of kombu releases even more umami flavor into the stock, and it is compact and easy to store, as well.

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On a properly cold day, I like to prepare a traditional Japanese wintertime dish that is one of the best ways I know to appreciate kombu.

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First, the primary dashi, which functions as this soup's broth, comes together when a pot of water with kombu is boiled and bonito flakes are tossed in.

WSJ: Amaryll Schwertner's Poached Halibut in Dashi With Leeks, Sweet Potatoes and Sesame Seeds | Slow Food Fast

Umami, the savory "fifth taste" that gives foods like anchovies and shiitake mushrooms their meaty depth, is found in kombu in a greater concentration than in any other food.

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Now for what Ms. Schwertner calls the secondary dashi, which serves as the poaching liquid for the halibut: Add the reserved bonito flakes and kombu to a pot with water and Meyer lemon slices.

WSJ: Amaryll Schwertner's Poached Halibut in Dashi With Leeks, Sweet Potatoes and Sesame Seeds | Slow Food Fast

To make this dashi, simmer kombu (a dried seaweed) and bonito (dried tuna belly flakes). (Both ingredients are available at most Japanese markets and some specialty stores.) If you can't find kombu, Ms. Schwertner suggests using dried shiitake mushrooms, which impart a similar umami flavor.

WSJ: Amaryll Schwertner's Poached Halibut in Dashi With Leeks, Sweet Potatoes and Sesame Seeds | Slow Food Fast