Preheat oven to 350°.
Remove white papery skin from garlic head (do not peel or separate the cloves). Wrap head in foil. Bake at 350° for 1 hour; cool 10 minutes. Separate cloves; squeeze to extract garlic pulp. Discard skins.
Heat oil in a large saucepan over medium-high heat. Add leek; sauté 5 minutes or until tender. Stir in garlic, squash, 2 cups water, broth, salt, and black pepper; bring to a boil. Reduce heat, and simmer 10 minutes or until squash is tender. Place half of squash mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over the opening in the blender lid (to avoid splatters). Blend until smooth. Pour pureed soup into a bowl. Repeat procedure with remaining squash mixture.
Wine note: The creamy texture of the pureed soup matches well with a plump but crisp Chardonnay like J.J. Vincent Bourgogne Blanc ($15). It's suitably rich, round, and fruity, with gently buttery flavors and good acidity to balance the natural sweetness of the squash. -Jeffery Lindenmuth