Slice brown onion thinly. Cut pork belly strips into halves or so to fit into a pot you use.

Put onion slices and pork in a pressure cooker and pour water to submerge. Bring to a boil at high heat. Put the lid on, lock, and build pressure. After the pressure is on, turn down to low heat and cook 30 minutes or until meat becomes very tender. Let the pressure release naturally. (Following the directions for your own pressure cooker, of course.) If you don't have a pressure cooker, use a large heavy pot such as a dutch oven and cook for 3 hours until meat is very tender.

Remove pork from the pot, and cut into 12 pieces or so. (Discard the brown onions.) Heat a wok with oil and green onions, and take the green onions out when browned. (Discard the green onions.) Cook pork pieces in the wok until surfaces are slightly browned. Add Soy Sauce, Sake and sugar. Put boiled eggs in the pan and cook until the sauce thickens, stirring occasionally, about 10-15 minutes.

Serve with Japanese yellow mustard.