Put the broth, ginger, roughly chopped scallions and 2 cups water in a medium saucepan and bring to a boil over high heat. Cover the pan, lower the heat so the soup just simmers and cook for 20 minutes.
Meanwhile, bring a large pot of water to a boil. Put the sliced scallions, watercress, tomatoes, avocados and cucumbers in separate small bowls and set aside.
Stir the edamame into the boiling water and cook for 1 minute. Add the soba noodles to the pot with the edamame and cook until the noodles no longer have a hard, chalky bite but are still springy, 3 to 4 minutes. Drain the noodles and edamame in a colander; shake to remove as much water as possible. Divide among 4 soup bowls.
Remove the broth from the heat and stir in the sherry and soy sauce. Ladle the hot broth over the noodles. Serve with the sliced scallions, watercress, tomatoes, avocadoes, cucumbers and chili garlic sauce for topping.
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