Place the meat in a large ziplock. Pour over the shio koji, seal the bag and shake well to cover the meat. Refrigerate for 24 to 48 hours.
Preheat your sous vide water bath to 142F. Lower the bagged meat into the water and cook for 18 up to 24 hours.
An hour before the meat is done, melt the butter into a heavy-bottomed pan with a lid. Add the onions, sprinkle them with salt and cook covered over low heat until they begin to wilt. Uncover and cook until onions are soft and very sweet.
When the meat is done. Remove it from the marinade. You can slice and serve it as is or sear it for a few minutes over high heat until it browns. Strain the marinade and pour it into a small saucepan. Reduce over medium-high heat until it thickens—season to taste.
Spread the stewed onions on a large serving platter, top it with sliced beef and pickled chili. Drizzle it with the sauce or serve it on the side.