Heat 2 tablespoons oil in a large Dutch oven over medium heat. Add the onion and cook, covered, stirring occasionally, until tender, 6 to 8 minutes.
Meanwhile, combine the bouillon base with 6 cups boiling water, stirring to dissolve.
Add the squash, carrots, 2 teaspoon turmeric and ½ teaspoon pepper to the Dutch oven and cook, stirring, 1 minute. Add the broth, bring to a boil, then reduce heat and simmer until the vegetables are very tender, 18 to 22 minutes.
Meanwhile, heat oven to 375°F. Toss the reserved seeds (about ¼ cup) with the remaining teaspoon of oil, ¼ teaspoon turmeric and ¼ teaspoon pepper and roast until golden brown and crispy, 9 to 11 minutes.
Using an immersion blender (or standard blender, working in batches) purée the soup. Sprinkle with the toasted seeds and swirl in the coconut milk.
