Bruise the lemongrass stalks with the back of a knife, then place them in a large pot with the ginger, garlic, and onion.
Add the low-sodium vegetable broth and bring the mixture to a gentle simmer over medium heat. Cook for 10 minutes to let the lemongrass flavor the broth.
Stir in the carrots, red bell pepper, and mushrooms. Simmer for 5 minutes.
Add the zucchini and pour in the light coconut milk. Continue simmering for another 5 minutes until the vegetables are tender but still hold their shape.
Remove the lemongrass pieces. Stir in the lime juice just before serving.
Ladle the soup into bowls and top each serving with chopped cilantro.