Add the chilies, shallots, garlic, ginger, cilantro stems, coriander, turmeric, curry, and ¼ cup water to a food processor. Blend on high until smooth, stopping to scrape down the sides as needed, about 1-2 minutes.
Add coconut oil to a large heavy-bottomed pot over medium-high heat. Add the puree and cook, while stirring constantly, for about 4 minutes to bring out the flavors.
Add the coconut milk, vegetable broth and butternut squash, then bring to a boil. Reduce to a low simmer and cook until the squash is tender, about 15 minutes.
Add the fish sauce and sugar, then taste and season with Kosher salt as needed. Stir in the spinach, then cover and simmer for another 5-10 minutes.
In the meantime, cook the noodles according to the package directions, then rinse them with cold water and keep them on the side (they will overcook and turn mushy if you add them directly to the pot).
Ladle the soup into bowls, then add a handful of rice noodles. Serve immediately with red onion, lime wedges and chili flakes on the side.