Saute the mushroom caps in the vegan butter for a few minutes over medium heat. Place the mushrooms over a strainer and press on it to drain most of the moisture out. When you are ready to assemble the wontons, follow these seven steps for folding wonton dumplings.
To make the broth
Start by gently sauteing the ginger, garlic and lemongrass hearts in a bit of oil in a medium-sized sauce pan for about 5 minutes over medium-low heat. Add in the stock, hollow lemongrass husks, vegan fish sauce, soy sauce and sugar. Stir well and allow to simmer over the lowest heat possible for 10 to 15 minutes.
Remove and discard the lemongrass husks and increase the heat to medium. Once it is at a very low boil, drop in the wonton dumplings, making sure not to overcrowd the saucepan with them. If the boil increases at all, dial down the heat a bit to ensure the boil stays consistent. After three to four minutes, remove the dumplings and place them in serving bowls.
Repeat the process until the dumplings have been prepared, then ladle the hot soup over them.
Sprinkle with the green onion and serve immediately.