Place first 5 ingredients in a saucepan and simmer covered for about 1 1/2 hours, or until squash is very tender. Cool 10 mins.
Puree solids with some of the liquid in a food processor or blender. Reserve the remaining liquid.
Return pureed squash to pan with the remaining cooking liquid and thin soup out with about 1 cup water. Add water gradually until you get the right thickness. You want a consistency thinner than a puree without being watery.
Reheat soup on very low heat. Taste for salt and add a little ground pepper.
Serve topped with minced sage leaves and optional dollop of crème frâiche.