Cook the bacon in a large skillet until crispy. Transfer the crispy bacon to a paper towel-lined plate to drain off the excess grease. Cool, and then chop or crumble the bacon into bite-sized pieces.
To the bacon drippings in the skillet, add the vinegar, honey, Dijon, salt, and pepper. Whisk to combine, and then remove from the heat.
Add the spinach, egg, onion, cheese, croutons, and chopped bacon to a salad bowl and toss gently to combine.
Drizzle the hot dressing on top. Toss again. Serve immediately.
Notes
Thick-cut bacon provides the best flavor and texture for the dressing.
Pour the dressing over the salad while it is still warm to lightly wilt the spinach.
Baby spinach works best because it is tender and mild in flavor.
If blue cheese is not preferred, feta or goat cheese can be substituted.
To mellow the flavor of red onion, soak the slices in cold water for 5 minutes before adding to the salad.
The salad should be served immediately after dressing is added.