Sauté the aromatics: Heat oil in a large pot over medium. Add onion with a pinch of salt and cook 4–5 minutes until soft. Stir in garlic and ginger; cook 30 seconds until fragrant.
Toast the spices: Add red curry paste and turmeric.
Cook 1 minute, stirring, to wake up the flavors.
Add sweet potatoes and liquid: Stir in sweet potatoes, vegetable broth, and coconut milk. Bring to a gentle boil.
Simmer: Reduce heat, cover slightly, and simmer 15–20 minutes, until the sweet potatoes are very tender.
Blend: Use an immersion blender to puree until smooth. Or carefully transfer to a blender in batches.
If you like more texture, blend only half.
Balance the flavors: Stir in lime juice, a small drizzle of maple syrup if needed, and season with salt and pepper. Adjust curry paste or chili for heat.
Serve: Ladle into bowls and top with cilantro or scallions. Add chili flakes and extra lime to taste.