Cook bacon in a large skillet until crisp; remove bacon, and drain on paper towels, reserving 1/4 cup drippings in skillet. Crumble bacon, and set aside.
Combine vinegar, mustard, and, if desired, 1/2 cup crumbled blue cheese, stirring until mixture is smooth.
Add vinegar mixture and onion to hot bacon drippings in skillet over medium heat, stirring to coat. Add bacon, parsley, salt, and pepper. Add spinach and endive; toss in skillet just until combined. Serve immediately. Top with additional crumbled blue cheese, if desired.
*1/2 (10-ounce) package fresh baby spinach may be substituted.