Prepare the ingredients: Thoroughly rinse the mushrooms and slice them. Cut the medium-firm tofu into thin strips that are about half an inch in thickness. Slice the green onions for garnish.
Make the soup base: In a pot, add water, rice vinegar, dark soy sauce, and chicken stock cube. Simmer on low heat until the chicken stock cube has dissolved.
Prepare two small bowls: One bowl is to make the roux, and the other is to whisk the eggs in. For the roux, transfer two ladles of soup base into a bowl and then add flour. Thoroughly stir to mix until the flour has dissolved, and pour the mixture back into the pot. Then, whisk the eggs in a separate bowl and keep them aside.
Cook the ingredients: Add the mushroom and the tofu to the pot and bring the soup to a boil. Set the temperature to medium-high and continue to cook for twenty minutes. Then, slowly pour the whisked eggs evenly all over the soup. Stir gently so that the eggs will spread out and cook without clumping.
Garnish and serve: Sprinkle the green onions and serve while hot.