In a medium pot heat up butter and oil over high until butter melts.
Add chicken broth and bring to a boil.
(Parallel to step 2) In a small bowl beat an egg and salt until the egg yolk and egg white are completely integrated.
Lower the temperature to low. Slowly pour the beaten egg into the broth while whisking (1-2 min). Cook another 3-4 minutes.
Garnish with parsley and serve.
Notes
Substitute chicken broth with vegetable or beef broth, or bouillon. If using bouillon use 2 teaspoons of powder, or 2 cubes, per 4 cups of water. Furthermore, you can substitute parsley with green onion, or omit it completely.
For a thicker soup make a light roux. This is done by adding a tablespoon of flour to the melted butter and whisking it together until it turns dark yellow. Then add the broth.
It’s best to consume this soup within a few hours of making it as the egg changes consistency after a while. Otherwise, keep it in the fridge and bring it to a boil before serving. Be aware the taste won't be the same as when you first made it.