Pour vegetable stock into a saucepan. Add the sautÈed onions and simmer, covered, for about 30 minutes, until onions are very soft.
Remove saucepan from heat and transfer mixture to a blender. Break block of soft tofu into pieces and add to soup in blender. Process until smooth and creamy, about 2 minutes.
Pour soup into bowls and garnish with a dash of nutmeg. Serve hot or chilled.