Dashi, a broth made with seaweed and shavings from a hunk of dried fish, lends intense flavor to everything it touches—from classic Japanese food to some of America's most ambitious restaurant dishes.

Look up umami in the dictionary and dashi is what you'll find. It tastes as rich and complex as a broth or stock that's been simmering for hours, but it takes less than 15 minutes to make and, in many ...