Cook the chopped bacon in a skillet over medium heat until crisp. Transfer the bacon to a paper towel-lined plate.
Reserve about 3 tablespoons of the bacon drippings in the skillet and discard the rest.
Add the minced shallot to the skillet and cook for 30 to 60 seconds until softened.
Stir in the apple cider vinegar, sugar, Dijon mustard, salt, and black pepper. Warm the dressing gently for about 1 minute, stirring until combined.
Place the spinach in a large serving bowl.
Pour the warm dressing over the spinach and toss gently until the leaves are lightly coated and just slightly wilted.
Top with the crispy bacon. Add sliced hard-boiled eggs, red onion, or mushrooms if using.
Serve immediately while the dressing is still warm.