Break each egg into a small bowl or a cup.
Bring the vinegar and 1 liter (approximately 1 quart) of water to a simmer in a small pot. Slide each egg into the water carefully. Swirl the the water around each egg to ensure that the egg white fully coats the yolk uniformly.
Cook the eggs for 4 minutes, until the egg white is firm and the yolk is still soft. Carefully lift the eggs from the water with a skimmer, and place on a bed of the spinach.
Season to taste with salt and pepper, and serve.