Preheat the oven to 350 degrees F. Roast the pumpkin until very tender, about 40 minutes. When cool enough to handle, scrape the flesh from the skin and discard the skin. Reserve 2 cups of pumpkin flesh.
Combine the vegetable stock, onion, garlic, ginger and lemongrass in a 4-quart saucepan and simmer for 10 minutes. Turn off heat, cover pan and let steep for 1 hour. Strain through a fine mesh strainer or cheesecloth, pressing to remove all the liquid. You should have about 2 cups of stock.
Wipe out the saucepan. Add the reserved 2 cups pumpkin flesh, coconut milk, and 1 cup of the stock to the saucepan and bring to a simmer.
Meanwhile, combine curry paste with 1/2 cup of the remaining stock in a bowl and stir until smooth. Add the curry mixture to the soup. Simmer the soup for about 20 minutes, adding more stock if the soup is too thick.
Add the fish sauce and lime juice. Taste for seasoning and add more fish sauce, lime juice, salt or pepper to taste. You can also add a squirt of sriracha if more heat is desired.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.