Preheat the oven to 425 degrees F. Toss the potatoes with the olive oil, balsamic vinegar, and season with salt and pepper. Spread in a single layer on a foil-lined baking sheet and roast until well browned and crisp, about 30 minutes. Let cool slightly.
Meanwhile, in a large skillet over medium heat, cook the bacon until very crispy. Transfer the bacon to a paper towel-lined plate to drain and then crumble and set aside. Discard all but 2 tablespoons of the bacon fat. Increase the heat to medium high and add the sliced mushroom. Cook until browned and tender and then season with salt and pepper.
For the vinaigrette:
Whisk together the balsamic vinegar, honey, Dijon, and garlic in a bowl. Whisk in the oil in a slow steady stream until an emulsion forms, then season with salt and pepper.
To assemble the salad, place the baby spinach leaves in a very large bowl and top with the crumbled bacon, sauteed cremini mushrooms, roasted potatoes, sliced onion, and Parmesan cheese. Dress lightly with vinaigrette, using about half, and toss to combine. Serve the extra vinaigrette on the side.
Cook's Note: The delicious combination of tender spinach leaves, crisp salty bacon bits, and tangy Parmesan cheese are offset with the sweet Dijon vinaigrette.