Heat oven to 300°F. Place bread cubes in single layer on ungreased cookie sheet. Generously spray cubes with nonstick cooking spray; sprinkle with Italian seasoning. Bake at 300°F. for 15 minutes.
Meanwhile, in blender container, combine 3/4 cup buttermilk, mayonnaise, salt and garlic; blend until smooth. Through center of blender lid, slowly add oil, blending just until combined. If necessary, add up to 1/4 cup additional buttermilk for desired dressing consistency. Refrigerate until serving time.
Just before serving, place lettuce and onion in large salad bowl. Add dressing to salad; toss gently to coat. Add croutons and cheese; sprinkle with pepper. Toss gently.