Thinly slice the onion and cut vermicelli noodles into manageable lengths using kitchen scissors.

In a pot, combine the onion and seasoning ingredients (salt, pepper, soy sauce, chicken stock, and water). Heat over medium heat until boiling.

Add the vermicelli and dried wakame to the pot. Cover and simmer on low heat for 2 minutes until the vermicelli softens and the onion becomes tender.

Increase to medium heat and bring to a boil. Slowly pour in the beaten egg, allowing it to set before turning off the heat. Drizzle sesame oil over the soup.