Gyudon restaurants are categorized as a fast-food restaurant. It is always served within one to three minutes. The rice is topped with thin slices of fatty beef seasoned with soy sauce, which Japanese really like. Gyudon originated in Tokyo Fish Market in 1899. It started as a simple and quick meal for the busy workers in the fish market. The concept behind gyudon was to provide a delicious meal that is cheap and served quickly. The concept is carried for many years until now. When I order a gyudon in a gyudon restaurant, I always order it with a raw egg. The combination of the beef and egg's flavor complements each other. On the picture, the other bowl is called pork miso soup, a common soup in Japan.
Sukiya web https://www.sukiya.jp/en/
Yoshinoya web https://www.yoshinoya.com/en/
Matsuya web https://www.matsuyafoods.co.jp/english/menu/