Heat the oil in a large soup pot over medium heat. Add the onion, garlic, and ginger and cook, stirring frequently, until softened, 6 to 8 minutes. Do not brown; reduce heat if necessary.

Add the squash, carrots, broth, fish sauce, sugar, and 2 tablespoons of the curry paste. Bring to a boil, then lower the heat and simmer, covered, until the vegetables are tender, about 20 minutes.

Using a hand-held immersion blender, purée the soup until silky smooth. (Alternatively, cool the soup slightly, then purée in a blender in batches, making sure to leave the hole in the lid open to allow the steam to escape.) Stir in the coconut milk, lime juice, and salt (if the coconut milk is solidified, use the immersion blender to mix it in). Bring to a simmer, then taste and adjust the seasonings, adding the remaining tablespoon of curry paste if more heat is desired. Ladle the soup into bowls. Drizzle with the Sriracha and garnish with the scallions, peanuts, and cilantro, if using.

Note: Check out easy guidance on how to peel, grate, and chop fresh ginger here.

Make Ahead: This soup can be made up to 3 days ahead of time and stored in a covered container in the refrigerator. Keep in mind that the soup will thicken up quite a bit as it cools, so you'll need to add a bit of water or broth to thin it back to the right consistency upon reheating.

Freezer-Friendly Instructions: The soup can also be frozen for up to 3 months. Defrost the soup in the refrigerator for 12 hours and then reheat it on the stovetop over medium heat until hot. (It may thicken up a bit while in the freezer; if so, just thin it out with a bit of water or broth while reheating.)