In a large skillet (1o-inch diameter works well), cook the bacon until crisp, and then remove to a plate lined with paper towels. Place 1 1/2 tablespoons of the bacon grease in a small bowl, and add the vinegar, sugar, Dijon, garlic, salt and pepper. Mix well; set aside.
Over medium high heat, sauté the onions in the remaining bacon grease until softened. Add the mushrooms and continue to cook, stirring regularly, until the mushrooms have released their moisture and are nicely browned.
Add the vinegar mixture to the pan, using it to deglaze the pan. Stir to evenly coat the vegetables with the dressing, and cook for another minute or until the garlic is fragrant. Remove from the heat.
Meanwhile, in a separate pan, cook the eggs to your liking. I like a sunny side up egg, but I put a lid on the pan for a few minutes so the top of the egg isn’t too goopy. Over-easy and poached eggs are good options, too.*
Assemble salad on individual plates by creating a base of spinach, and then topping with the mushroom mixture, an egg, crumbled bacon and blue cheese. Season with a little extra salt and pepper and enjoy!