Fry the bacon in a skillet until crispy. Remove to a paper towel. Drain the grease into a bowl & set aside. Cook the sliced onion in the skillet slowly until caramelized and reduced.
Chop the bacon. Peel & slice the eggs.
Make the hot bacon dressing. Mix 3 tablespoons of reserved bacon fat, vinegar, sugar, Dijon mustard, & salt to a small saucepan or skillet over medium high heat. Whisk together & heat thoroughly until bubbly.
Add the spinach to a large bowl. Arrange the onions & bacon on top. Pour the hot dressing on, toss to combine, & then top with the egg slices.