Complex sentence: It is also a popular restaurant food, often served in restaurants operated by retired sumo wrestlers who specialize in the dish; the first of these, Kawasaki Chanko, was started in 1937 in the Ryōgoku district of Tokyo, home to many prominent sumo stables.
Split into simpler sentences: It is also a popular restaurant food, often served in restaurants operated by retired sumo wrestlers who specialize in the dish. The first of which, Kawasaki Chanko, was started in 1937 in the Ryōgoku district of Tokyo, home to many prominent sumo stables.