Sauté onion in vegetable oil in a 4-quart pot until translucent.
Peel squash and cut into 1-inch cubes. Add to the onion and cover with 5 cups water or vegetable stock. Bring to a boil, then reduce heat and simmer covered for about 30 minutes or until squash is soft.
Let soup cool slightly for easy handling. Purée in batches in a blender or food processor, adding tahini during the last batch. Return to soup pot after puréeing.
Season with tamari and salt to taste. Simmer 10 minutes.
Garnish with croutons, if desired.