Cook bacon in a large skillet until crisp; remove bacon, and drain on paper towels, reserving 1 teaspoon drippings in skillet. Crumble bacon, and set aside.
Sauté onion in hot drippings in skillet over medium heat 10 minutes or until tender and lightly browned. Remove from heat; cool.
Process buttermilk and next 4 ingredients in a blender or food processor until smooth, stopping to scrape down sides. Stir in onion.
Top each lettuce wedge with dressing; sprinkle with bacon, and top with shredded basil, if desired.
* Turkey bacon may be substituted.