In a large soup pot or Dutch oven, heat coconut oil over medium high heat. Once hot, add the chopped shallot and onion and cook for approximately 2 minutes, or until just turning soft, stirring frequently. Add the ginger, garlic, lemongrass, and jalapeno and stir. Lower heat to medium and cook for approximately 5 minutes, or until fragrant and just starting to turn golden; continue to stir frequently to prevent burning. Add the yellow curry powder, turmeric, cayenne powder and salt and stir well. Still stirring, cook for an additional minute or two and add the vegetable stock.
Turn heat to high and bring pot to a boil. Carefully add the sweet potatoes, carrots and pureéd pumpkin. Reduce heat to low and simmer, uncovered, for approximately 20 minutes, or until potatoes are fork tender.
Pureé soup until very smooth, being careful not to overfill your blender with hot, boiling, steaming soup (I usually only fill mine half way full). I usually do mine in several small batches. Return to the stove over medium heat.
Gently stir in the soy sauce, fish sauce (optional, but HIGHLY recommended), brown sugar and fresh lime juice. Taste and adjust seasonings, as necessary. There is no magic formula here, so play around until you find what balance suits you.
Finally, stir in the coconut milk. Some may argue to do this before you add the soy sauce, fish sauce and sugar, however, I always add it last. I added 1/2 cup coconut milk to this pot of soup, but feel free to add more for a creamier, richer soup if that's what you desire (my husband prefers more coconut milk, so he will usually drizzle a tablespoon or two over the top of his bowl at serving).
Garnish with chopped pistachios, chopped fresh cilantro or green onion, lime juice or coconut milk.