Combine the American Wagyu ground beef, Worcestershire sauce, salt and pepper, smoked paprika, and Blazing Star Beef Rub. Gently mix until everything is evenly incorporated.
Form the beef into small, loosely packed balls and place them in the refrigerator while you prepare the gravy.
Melt the butter in a skillet over medium heat. Add the sliced onion and sauté until softened and lightly caramelized.
Sprinkle the cornstarch over the onions and mix until everything is evenly coated.
Slowly pour in the beef broth while stirring to prevent lumps.
Add the shoyu, Worcestershire sauce, optional garlic chili oil, and ketchup.
Continue cooking and stirring until the gravy has thickened. Season to taste as needed.
Fire up your griddle until it is ripping hot.
Place the chilled beef balls onto the griddle and smash them into patties.
Cook until a deep crust forms, then flip and finish cooking on the second side.
Build each loco moco with a mound of white rice, Wagyu smash patties, plenty of onion gravy, and a fried egg.
Finish with sliced green onions and enjoy!