Step 1Peel the squash, using a sharp vegetable peeler. Cut it in half lengthwise and scoop out the seeds with a spoon. Cut in 1-inch chunks.
Step 2Heat the butter in a large soup pot over medium heat. Add the onion and cook until it softens, about 5 minutes. Add the garlic and ginger root and cook until fragrant, about 3 minutes. Add the diced squash, water and salt and bring to a simmer. Cook at a low simmer until the squash is tender enough to smash with a spoon, about 30 minutes.
Step 3Puree the soup, either using an immersion blender or grinding it in several stages in a regular blender. The soup should be completely smooth and a little thicker than heavy cream. Ladle the soup through a strainer into a clean soup pot, discarding any bits of squash or ginger left behind.
Step 4Heat the soup through once more and season to taste, adding salt first and then the vinegar. Add the vinegar one-half teaspoon at a time; it will take a little more than 1 tablespoon.
Step 5To serve, stir the creme fraiche to loosen the texture. Ladle the soup into wide bowls and spoon 1 to 2 tablespoons of creme fraiche in a decorative pattern onto each. Scatter a few toasted slivered almonds over the soup and serve immediately.