Cook bacon according to your preference. Reserve bacon fat when finished cooking.
Bring 2 quarts of water to a boil in a small pot. Separately, prepare a small bowl of ice water. Gently add eggs to the boiling water and cook: 6.5 minutes for soft boiled, or 8 minutes if you prefer hard boiled. When finished cooking, drain and transfer eggs to the ice water to stop the cooking, and make them easier to peel. Once cooled, peel eggs, cut each in half and set aside.
Place 4 Tbsp of the reserved bacon fat in a small bowl. Add vinegar and whisk until the mixture becomes thick and cloudy - your vinaigrette is now emulsified.
Wash and dry fresh spinach. Toss spinach with the vinaigrette, then divide into 2 large serving bowls. Crumble bacon over the spinach, and top each salad with eggs.