Preheat oven to 350°.
Remove white papery skin from garlic head (do not peel or separate the cloves). Rub 1 1/2 teaspoons oil over garlic head; wrap in foil. Bake at 350° for 1 hour; let cool 10 minutes. Separate cloves; squeeze to extract garlic pulp. Discard skins. Set garlic pulp aside.
Heat remaining 1 tablespoon oil in a large Dutch oven over medium heat. Add onion and leek; cook 30 minutes, stirring often. Add 1/2 teaspoon salt and thyme. Cook an additional 30 minutes or until onion is golden, stirring occasionally. Stir in flour. Add wine and broth; bring to a boil. Reduce heat; simmer 30 minutes. Add garlic pulp, remaining 1/2 teaspoon salt, and milk to onion mixture; simmer 8 minutes or until thoroughly heated.
Place half of onion mixture in a blender, and process until smooth. Pour puréed mixture into a large bowl; repeat procedure with remaining onion mixture. Garnish with sour cream and caraway seeds, if desired.