Place the bacon in a medium nonstick skillet and fry over medium heat, stirring occasionally, until crisp, 8 to 10 minutes.
Make the dressing: in a large bowl, whisk together the vinegar, honey, mustard, salt, and pepper. Set aside.
Using a slotted spoon, transfer the cooked bacon to a paper towel–lined plate.
Transfer the bacon fat to a heat-proof bowl. Measure 4 tablespoons of bacon fat, return 4 tablespoons bacon fat to the skillet. Add the shallots to the skillet and cook over low heat, stirring frequently, until softened, 1 to 2 minutes.
Add the bacon fat and shallots from the skillet to the vinegar mixture and whisk to combine. Add the spinach and toss to coat evenly. Taste and adjust the seasoning with salt and pepper, if necessary. Divide the salad between 4 bowls; evenly divide egg and cooked bacon among them. Top with croutons if using. Serve immediately