Prepare the lemongrass: Trim the tough, dark green ends off the lemongrass stalks, leaving the lighter-colored, lower sections (about 3 or 4 inches). Bruise each stalk using the back of a knife or a meat pounder to release the essential oils, and slice them in half lengthwise.
Sauté aromatics: In a large pot, heat the oil over medium-high heat. Add the bruised lemongrass stalks and sauté until fragrant, about 3 minutes. Add the onion and sauté for about 5 minutes until softened. Stir in the garlic and ginger and cook for another minute until fragrant.
Simmer the broth: Pour in the vegetable broth and add the lime leaves. Cover the pot with a lid and bring to a boil. Reduce the heat to medium-low and let it simmer for at least 20 minutes to allow the flavors to infuse fully.
Add vegetables: Remove the lemongrass stalks and lime leaves from the soup base. Add the mushrooms and Bok choy. Simmer uncovered for about 5 minutes, or until the vegetables are tender.
Finish the soup: Remove the pot from the heat. Stir in the coconut milk, fresh lime juice, and coconut sugar (if using). Season with salt to taste.
Serve: Ladle the soup into bowls and garnish with fresh cilantro leaves and chopped scallions