Wash and clean the spinach. Peel and finely chop the onion. Boil 750 ml water in a pot, add vinegar. Beat the egg in a cup and let it slide into the boiling water. Cook over low to medium heat for about 5 minutes.
Heat the oil in a frying pan. Sauté the onion in it. Add spinach dripping wet and cover and let it collapse. Mix in pesto and yoghurt, season with salt and pepper. Roast the toasti golden brown in a toaster.
Remove the egg from the water with a skimmer and let it drain on kitchen paper. Spread the spinach mixture on the toasti. Place the egg on top. Sprinkle with pepper and thyme.