Loco Moco is a popular Hawaiian dish that consists of a hamburger patty topped with a savory brown gravy, topped with a sunny side up egg, and served over white rice.
Gently combine all the beef patty ingredients in a medium sized bowl until well combined. (Don't overmix or it can make them tough).
Using your hands, form the meat into even 1 inch thick patties. Set aside on a plate.
Heat a cast iron skillet over medium high heat until hot. Add patties so they don't touch each other (you may have to cook in a couple of batches). Cook 5 minutes each side, or until desired doneness.
For the Gravy
Add the butter, mushrooms, and onion to the hot pan. Season with garlic powder, salt, and pepper. Cook until soft, about 5 minutes.
Next, add flour, stir to incorporate and cook for a minute. Slowly add your beef broth, stirring frequently. Season with both soy sauces and msg. Bring to a low boil and reduce the heat to simmer. Stir frequently. Taste and season as needed. After several minutes, it should have thickened.
To assemble, spoon a layer of rice onto a plate, followed by a patty, topped with gravy, and finished with the sunny side up egg. Garnish with green onion if desired.