[2] Seolleongtang is typically simmered over a low flame over a period of several hours to an entire day, to allow the flavor to be gradually extracted from the bones.
[3] In the Joseon dynasty, Koreans regularly made nationwide sacrifices to their ancestors, such as Dangun (the legendary founder of the kingdom of Gojoseon).
The nationwide sacrifice was called Sŏnnongje (hangul: 선농제; hanja: 先農祭, Sŏnnong meaning "venerated farmer"), and the altar for the sacrifice was called Sŏnnong dan (hangul: 선농단; hanja: 先農壇), which dates back to the Silla Dynasty.
To increase the food supply in Joseon, King Seongjong ordered them to invent dishes that could feed the maximum number of people using the fewest ingredients, and seonnongtang (tang meaning "soup") was one of these.
Mongolian food "Sulen" is sliced and boiled beef with green onions, which developed into seolleongtang in Korea.