Over medium-high heat, cook onions in oil until onions are clear-colored. Add broth and sweet potato, boiling until potatoes are soft. Then add carrots, ginger and garlic; reduce heat and simmer until carrots are tender when pierced.
Remove from heat and transfer to a food processor, you may have to divide the soup into two or three servings to blend it…I often overfill the food processor and end up with soup everywhere! =) Pulse the ingredients several times then puree until smooth. Add lemon juice before serving and garnish with fresh herbs.