How to make it
In a bowl, mix ground beef, black pepper, 3 Tbsp. hoisin sauce and 1 Tbsp. soy sauce. Form mixture into 4 equal patties, 4 inches in diameter.
In a skillet, add 1 Tbsp. oil, sautee onions and mushrooms and 1 Tbsp. soy sauce. Cook for 5 minutes and set aside.
Cook patties in a non-stick skillet over medium-high heat. Cook patties thoroughly.
Assemble burger patties with onion mushroom mixture.