In a 10-inch skillet, heat the oil on medium. Add the onion and ginger. Season with salt. Cook 5–7 min., until onions begin to soften, stirring often. Add the garlic and curry powder. Cook 2 min., stirring often. Stir in ¼ cup broth, scraping up any browned bits. Transfer contents of skillet to a slow cooker bowl.
To slow cooker, add the carrots, sweet potatoes, and remaining broth. Season with salt. Cover and cook 6–8 hours on low, until carrots and sweet potatoes are very tender.
In batches in blender or with immersion blender, purée soup until smooth. Season with salt and pepper to taste. Stir in the lemon juice. Garnish with the green onions and serve with the baguette, if desired.