Preheat the oven to 325°F.

Combine the soy sauce, chicken stock, brown sugar, mirin, five spice powder, and garlic in a Dutch oven or other heavy pot with a lid. Stir until the sauce is well blended.

Place the beef short ribs into the sauce. Cover the pot and place it in the oven for 3 to 4 hours, stirring halfway through the cook time if possible.

When the meat is so tender it's falling off the bone, remove the ribs from the cooking liquid and set aside.

To finish the sauce, first drain off the excess fat. You may want to use a spoon to spoon off the fat or you may want to use a fat separator.

Place 2 cups of the remaining sauce into a small pot over medium heat.

Make a cornstarch slurry by combining the corn starch and water and mixing until smooth. Stir the cornstarch slurry into the sauce and stir until the sauce thickens.

You may want to debone the short ribs and shred the meat.

Serve with the sauce and rice if desired.