Place the eggs in a saucepan and cover with cold water by 1 inch. Bring to a rolling boil, then immediately turn off the heat, cover, and let stand for 10-12 minutes. Transfer the eggs to an ice water bath to cool completely, then peel and slice or quarter them.
While the eggs cook, cook the bacon in a large skillet over medium heat, turning occasionally, until brown and crispy, about 8-10 minutes. Transfer the bacon to a paper towel-lined plate, leaving about ¼ cup of rendered fat in the skillet. Once cooled, crumble the bacon.
Place the washed and dried spinach in a large salad bowl. Add the sliced red onion and mushrooms.
Gently reheat the bacon drippings in the skillet over medium-low heat. Whisk in the red wine vinegar, Dijon mustard, and sugar. Continue whisking until the dressing is smooth and just begins to simmer. Season with salt and pepper to taste, then remove from heat.
Pour the warm dressing over the spinach, onions, and mushrooms. Toss gently to coat the leaves; the spinach will wilt slightly.
Top the salad with the crumbled bacon and hard-boiled eggs. Serve immediately while the dressing is warm.