In a large saucepan over medium heat, heat the ghee or olive oil. Add the onion and sweat until translucent, about 8 minutes, stirring occasionally.
Add the carrots, ginger, and lemongrass and stir to coat the vegetables with the ghee or oil. Reduce the heat to medium-low and cook for 10 minutes.
Add the broth and increase the heat to medium. Bring the soup to a simmer and cook until the carrots are tender, about 15 minutes.
Transfer the mixture to a blender and blend until smooth, blending in batches if necessary. Season with sea salt to taste and serve immediately.
You can refrigerate the soup for up to 1 week, or freeze it for up to 6 months.
