In a 4-quart saucepan melt the butter over medium-high heat, and add and sweat the onions. Add squash and pears and sweat those too a bit. Pour in the stock, enough to submerge solids. Add sprig of rosemary and bring to a simmer and cook until squash is fork tender about 15 to 18 minutes. Remove rosemary. Puree with immersion blender. Add a touch of cream and season, to taste.