In a large pot, heat the olive oil over medium heat. Add the chopped onion and sauté for about 5 minutes until it becomes translucent.
Add the minced garlic and grated ginger to the pot, and sauté for an additional 2 minutes until fragrant.
Stir in the red curry paste and cook for 1 minute, allowing the flavors to develop.
Add the diced sweet potatoes and vegetable broth to the pot. Bring to a boil, then reduce the heat and let it simmer for about 15-20 minutes, or until the sweet potatoes are tender.
Remove the pot from heat and carefully blend the soup using an immersion blender until smooth. (Alternatively, transfer to a countertop blender in batches, but be cautious of hot liquid.)
Return the blended soup to the pot and stir in the coconut milk. Heat it gently on low until warmed through, then add the lime juice, and season with salt and pepper to taste.
Serve the soup hot, garnished with fresh cilantro and a sprinkle of red chili flakes for an extra kick.
Notes
For a spicier kick, adjust the amount of red curry paste and red chili flakes.