"Zastroika" is a traditional Bulgarian technique where you temper the soup broth with an egg yolk and milk mixture. To do this, prepare a separate bowl with one egg yolk and a bit of milk or plain yogurt. Take ladles of the spinach soup broth (one at a time). While whisking it into the egg and milk mixture off the heat, temper the egg. Once the egg is tempered, add this mixture back into the soup.