Place the pork in a large pot and pour enough water and bring to a boil over high heat. Turn the heat down and simmer it for 30 minutes.
In a small pot, place the sauce ingredients and boil until the alcohol of Mirin evaporates over the medium heat.
Place the boiled pork in a food safe plastic bag and pour the sauce over it. Seal it tight, then refrigerate overnight.
Steamed Bean Sprouts
Rinse bean sprouts and place in a microwave safe bowl. Sprinkle pinch of salt and dash of sesame oil. Cover the bowl with a plastic wrap and microwave for 1 minute.
Ramen
Cook the noodles and soup by following the cooking directions on back of the package.
Slice the homemade chashu and warm it up in a microwave for 20 seconds.
Top chashu on ramen together with other toppings, such as the steamed bean sprouts, ajitsuke egg, and spring onions. Enjoy!