Heat the olive oil in a large pot or Dutch oven over medium heat.
Add the chopped onion and sauté for 5 minutes until translucent.
Stir in the minced garlic and ginger, cooking for an additional minute until fragrant.
Add the red curry paste and cook for another 2 minutes, stirring frequently.
Add the cubed sweet potatoes to the pot and pour in the vegetable broth. Bring to a boil.
Reduce the heat and let it simmer for about 20 minutes, or until the sweet potatoes are tender.
Remove the pot from the heat. Using an immersion blender, blend the soup until smooth. (If using a standard blender, blend in batches; be careful with hot liquids.)
Return the blended soup to the pot. Stir in the coconut milk, lime juice, and soy sauce. Heat the soup gently until warmed through.
Season with salt and pepper to taste. Adjust lime juice or soy sauce if needed.
Serve hot, garnished with fresh cilantro, if desired.