Katsudon is Tonkatsu (deep-fried pork) and eggs cooked in a sweet and salty broth and placed over rice. Don (donburi) means a bowl, and Donburi dishes are a popular kind of casual rice dish in Japan. Because you have to prepare Tonkatsu first, it is a little bit of work involved since you cannot cook everything in one pan. Katsudon is a hearty dish compared to other Japanese food because Tonkatsu is breaded and deep-fried, but the taste is so good that you will not mind the extra calories from the oil. Besides, since deep-fried Tonkatsu is cooked in tasty broth and is crunchy yet juicy, you may not notice the grease at all (well, maybe, just maybe, calories might stay in, or near, your tummy).
