Chashu is basically the Japanese version of Chinese Cha siu.(Marinated pork in special sauce)
To try one of the best, I visited Daikokuya (Hacienda Heights, Ca.) They also have locations in Little Tokyo, Monterey Park, and Costa Mesa but those locations are known to have hellish waiting times.
Many rave about Daikokuya as having the number one chashu ramen in Los Angeles. That probably true, if not…it’s probably pretty damn close.