Although homemade dashi (Japanese seaweed broth) is part of the easy beauty of this dish, you can substitute instant dashi in a pinch. If you take that shortcut, infuse the broth with fresh ginger.
2 large sheets kombu (kelp)
2 quarts cold water
1 cup bonito flakes (about 1/4 ounce)
1 (1-inch) piece fresh ginger, peeled and thinly sliced
1 ounce dried mushroom blend
1 (14-ounce) package fresh jumbo udon noodles
2 green onions, trimmed
1 tablespoon sugar
2 1/2 tablespoons low-sodium soy sauce
2 tablespoons rice vinegar
1 pound medium shrimp, peeled and deveined
How to Make It
Wipe kombu clean with a cloth; cut sheeting lengthwise into 3-inch pieces. Place kombu in a large saucepan over medium-high heat; cover with 2 quarts cold water. Heat mixture to 180° or until tiny bubbles form around edge (do not boil). Reduce the heat to medium-low, and simmer 20 minutes. Remove from heat; discard kombu. Stir in bonito and ginger; let stand 10 minutes or until bonito sinks. Strain through a cheesecloth-lined colander over a bowl; discard solids.
Wipe pan clean. Return broth to pan; bring to a boil. Remove from heat. Stir in mushrooms; let stand 20 minutes. Strain mixture through a cheesecloth-lined colander over a bowl, reserving mushrooms and broth. Wipe pan clean. Return broth to pan; bring to a simmer. Coarsely chop mushrooms.
Cook noodles according to package directions; drain. Cut onions into 1-inch pieces; cut each piece in half lengthwise. Cut onion pieces lengthwise into thin strips. Combine sugar, soy sauce, and vinegar in a small bowl, stirring well. Add shrimp to broth; cook 1 minute. Stir in mushrooms; cook 30 seconds. Remove from heat; stir in vinegar mixture. Place about 1 cup noodles in each of 4 bowls; top with 2 cups broth mixture. Divide onion strips evenly among bowls.