Place eggs in a saucepan and cover with cold water. Bring to a boil over high heat. Once boiling, cover and remove from heat. Let stand for 12 minutes.
Transfer eggs to an ice-water bath to stop the cooking process and make them easier to peel.
In a bowl, combine soy sauce, water, rice vinegar, sugar, minced garlic, sesame oil, and red pepper flakes. Stir until the sugar is dissolved.
Peel the eggs and place them in a container that can fit all of them snugly. Pour the marinade over the eggs, ensuring they are fully submerged. Sprinkle chopped green onions and sesame seeds on top.
Cover and refrigerate for at least 4 hours, but preferably overnight for deeper flavor.
Slice the eggs in half and serve with a bit of the marinade spooned over the top. Optionally, garnish with more green onions or sesame seeds.
