Cook bacon in a pan or large skillet until crispy. Remove from the pan and drain on paper towel. Reserve all of the bacon drippings in the pan.
Fry onion in the bacon fat until soft.
Add in vinegar, sugar, salt and pepper. Cook on medium heat until the sugar dissolves.
Place spinach in a large salad bowl.
Pour the hot bacon dressing over the spinach. Toss to coat. The spinach should wilt under the heat of the dressing.
Crumble bacon and sprinkle over the salad before serving.