Udon Soup with Shrimp

While homemade dashi (Japanese seaweed broth) is part of the easy charm of this dish, as a last resort, you can substitute instant dashi instead. If you use this method, fill the broth with fresh ginger.
Wipe the kombu with a cloth; cut lengthwise into 3-inch pieces. Put the kombu in a large saucepan over medium-high heat; cover with 2 liters of cold water. Preheat the mixture to 180 ° C or until tiny bubbles form around the edges (do not bring to a boil). Reduce the heat to medium-low and simmer for 20 minutes. Remove from heat; discard the kombu. Add the mackerel and ginger; let stand for 10 minutes or until the mackerel is soaked. Strain through a cheesecloth-lined colander over a bowl; remove any leftovers.Wipe the pan dry. Return the broth to the pan; bring to a boil. Remove from heat. Add the mushrooms; let stand for 20 minutes. Strain the mixture through a colander lined with cheesecloth into a bowl, reserving the mushrooms and broth. Wipe the pan dry. Return the broth to the pan; bring to a boil. Chop the mushrooms coarsely.Cook the noodles according to the directions on the package; drain. Cut the onion into 1-inch-thick slices; cut each slice in half lengthwise. Cut the onion lengthwise into thin strips. Combine the sugar, soy sauce, and vinegar in a small bowl, mixing well. Add the shrimp to the broth; cook for 1 minute. Add the mushrooms; cook for 30 seconds. Remove from heat; add vinegar mixture. Place approximately 1 cup of noodles in each of the 4 bowls; top with 2 cups of stock mixture. Spread the onion strips evenly over the plates.