While squash is cooking in oven, add butter, onion, leek and garlic to a large stock pot and cook over high heat for a few minutes until gloriously fragrant.
Add 5 cups water and applesauce to pot. Reduce to a simmer and cover.
Scoop golden flesh out of the squash skins (don't burn yourself!) and add to pot.
After all the squash has been added, puree the mess with an immersion blender or regular ol' blender.
Stir in yogurt.
Add salt and pepp to taste.