In a big bowl, mix ground beef, Worcestershire sauce, seasoning salt, garlic powder, and black pepper.
Shape into thick patties, ensuring they are smooth on the edges. Expect some shrinkage during cooking.
Fry each patty in a pan over medium heat for 3-5 minutes on each side until well done. Place finished patties on a paper towel-lined plate to absorb excess grease.
Mix beef stock, shoyu, Worcestershire sauce, ketchup, and cornstarch in a bowl. Whisk until the cornstarch is well combined.
In the same pan used for patties, caramelize onions in one tablespoon of butter for a minute. Add mushrooms if desired.
Melt the remaining butter, pour in the gravy mix, and stir until it thickens and simmers. Melt a slice of butter in a frying pan.
Crack an egg into the pan and cook it sunny side up or as per your preference. Set aside the cooked egg.
Scoop a generous serving of hot white rice onto a plate. Place a hamburger patty on top and drizzle with the prepared gravy.
Crown the dish with the cooked egg and garnish with green onions. Serve your Loco Moco hot and savor the deliciousness!