In a large pot, heat the oil over medium heat. Add the onion and sauté for 5 minutes. Add the garlic, ginger, lemongrass and red curry paste and cook for 3 minutes.
Add the pumpkin puree and broth, stirring to incorporate, then season with salt, and bring to a boil.
Reduce the heat to low and simmer for 15 minutes. Turn off heat and stir in the coconut milk and lime juice and stir just to heat through.
Remove the lemongrass stalk if you used whole stalk). Transfer the soup to a blender (or use an immersion blender in the pot) and puree until smooth.
If using, add cooked shredded chicken with soup puree back into stockpot and heat through.
Serve warm and garnish with fresh cilantro and thinly sliced chiles.