Make the chicken filling: Add chicken thigh and garlic to a food processor. Pulse until finely ground.Transfer to a bowl and mix in vegetarian oyster sauce, soy sauce, green onion, white pepper, and egg until well combined.
Stuff the mushrooms: Fill each mushroom generously with the chicken mixture, smoothing the top.
Pan-fry: Heat oil in a pan over medium heat. Place mushrooms chicken-side down and cook for 3-4 minutes until golden.
Make the sauce: In a small bowl, mix soy sauce, chicken stock, mirin, and cornstarch until smooth.
Finish cooking: Flip the mushrooms and cook for 2 minutes. Drizzle in the sauce, making sure ½ of the mushroom is submerged with the stock, cover, and cook for another 4 minutes, until the chicken is fully cooked and the sauce thickens.
Serve: Spoon extra sauce over the mushrooms and enjoy hot.
