Place the spinach in a medium salad bowl. Set aside.
Cook the bacon in a skillet over medium heat, stirring occasionally, until
crispy, about 5 minutes.
Remove from the pan and place on paper towels to cool.
Remove the pan with the bacon grease from the heat. Stir in the vinegar and mustard and season with salt and black pepper to taste. Pour over the baby spinach and toss to coat.
Top with the reserved chopped bacon.
Notes
Sliced mushrooms or shredded Brussels sprouts can be added on top.Due to the dressing, it's recommended to store the salad and dressing separately, if not eaten right away.