In a 3- to 4-quart pan over high heat, bring 2 cups broth and juice to a boil.
Meanwhile, peel squash, seed, and cut into 1-inch chunks (you need 7 to 8 cups).
Add squash, onion, and whole sage leaves to pan, cover, and return to a boil. Reduce heat and simmer until squash is very tender when pierced, about 30 minutes.
With a slotted spoon, transfer about half the vegetables to a food processor or blender. Whirl to smoothly purée, adding just enough broth to facilitate. Pour purée into a bowl and repeat to purée remaining vegetables.
Return puréed vegetables to broth in pan. If soup is thicker than you like, thin with more broth. Stir over medium heat until soup is hot.
Ladle into mugs. Add crème fraîche, sage, and salt to taste.