Place the bacon in a cold skillet and turn the heat to medium. Cook, stirring frequently, until crisp, about 8-10 minutes. Remove the bacon with a slotted spoon and drain on paper towels.
Leave 3 tablespoons of the bacon grease in the pan, add the vinegar, and cook over medium heat for a minute.
Place the spinach in a serving bowl, add the dressing, and toss. Add the eggs and green onions and toss gently. Season to taste with salt and pepper. Serve immediately.
Notes
If using regular, unseasoned rice vinegar, add a teaspoon of sugar.