Prepare the lemongrass stalks by peeling off 1 or 2 layers of the dry outer leaves. Discard the very thin, dry parts of the stalk that look like twigs. Slice the lemongrass stalks into 4-inch sections. Take a kitchen mallet and bash the bottom sections of the lemongrass to release more flavor. Set the lemongrass aside.
Heat the oil in a large pot over medium heat. Add the onions, garlic, and ginger. Cook for about 4 minutes, stirring occasionally. Next, add the Thai chili and 1/2 teaspoon of salt, and cook for another minute.
Add the cubed butternut squash, vegetable broth, remaining 1/2 teaspoon of salt, and coriander, if using. Lay the pieces of lemongrass over the butternut squash, but make sure they are submerged under the broth. Placing the lemongrass here will make them easier to remove when the soup is done cooking.
Cover the pot and bring the broth to boil. Then, reduce the heat to low and simmer for another 10 to 13 minutes, covered. The soup is ready when the squash is fork tender.
Uncover the pot and let the contents cool for 10 minutes.
Use tongs to remove the lemongrass and discard. Add the coconut milk.
Scoop the contents into a high-speed blender and blend in smaller batches until smooth. Alternatively, you can use an immersion blender and blend the soup directly inside the pot.
Serve the soup in bowls. Garnish with lime juice, chopped scallions, toasted coconut, and toasted pumpkin seeds, if you like.