I've been getting this at a local Japanese place for months and finally tried making it at home. It turned out delicious and surprisingly close to what they make.
2lbs pork shoulder (I got one pre-sliced at an Asian market) Braise shoulder in: 1/2 cup soy sauce 1 cup mirin 1Tbsp sugar 8 cloves garlic, crushed 2" of ginger, sliced
Combine all in a heavy pot and heat on the stove until boiling then cover and put in the oven at 250 for 4-6 hours. Serve over sushi rice with sliced green onions and slivers of pickled root veg, if you have them.