Thinly slice the onion.

Cut the bread in half and make a pocket by slicing halfway through the thickness. Spread butter on the bread.

Heat salad oil in a frying pan over medium heat. Add pork and cook until the color changes. Add onion and sauté until softened. Add ketchup, Worcestershire sauce, and butter, then stir-fry until the liquid evaporates.

Stuff 1/4 of the pork mixture into each bread pocket. Repeat to make 4 sandwiches.