How to make it
Chop onions and mushroom into a quarter-inch dice.
Heat olive oil in a medium-size skillet over medium heat. When oil is shimmering add onions and cook until translucent, about 4 minutes. Add the mushroom and stir to incorporate. Cook until mushroom begins to soften, an additional 2 minutes. Transfer cooked mushroom and onions to a large bowl and allow to cool for 10 minutes.
Chop the cilantro. Add the cilantro and beef to the cooled onion-mushroom mixture. Using clean hands, mix well to incorporate all ingredients.
Form beef mixture into 4 even patties and place on a tray. Brush oyster sauce over both sides of the 4 burgers. Cover and refrigerate for at least 15 minutes.
Pre-heat a grill pan or grill to high heat. Coat the grill pan with nonstick cooking spray. When oil begins to shimmer, place the burgers on the grill pan or grill. Cook for 5 minutes on each side or until the burgers reach desired doneness.