Rice, a beef patty, fried egg, brown gravy — loco moco is Hawaii's post workout high calorie answer and it's one of the greatest rice plates ever invented.
In a large bowl, combine the ground beef, panko breadcrumbs, milk, 1 tbsp Worcestershire sauce, half of the diced onion, salt, and pepper; gently mix until just combined. Form into 4 equal patties, being careful not to overwork the meat for tenderness.
Heat a large skillet over medium-high heat. Add 1 tbsp butter and sear the patties for about 3–4 minutes per side until a golden-brown crust forms. Transfer patties to a plate (they will finish cooking later).
Lower the heat to medium, add remaining butter, then sauté the remaining diced onion and mushrooms until softened, about 5 minutes, scraping up any browned bits for maximum flavor.
Sprinkle in flour and cook for 1 minute, stirring constantly. Gradually whisk in beef stock, soy sauce, and the remaining Worcestershire sauce. Stir continuously until the gravy is thick and smooth—about 4–5 minutes. Return the patties to the gravy and simmer over low heat for another 5 minutes so flavors meld and patties cook through.
In a separate nonstick pan over medium heat, fry the eggs sunny-side up in a little oil or butter until whites are just set but yolks remain runny. Turn off the heat immediately to avoid overcooking.
To assemble, place 1 cup of hot cooked rice on each plate, set a beef patty over rice, spoon a generous amount of mushroom gravy over both, and top with a fried egg. Serve immediately.