Heat butter over medium heat in a large skillet.
Add onions and cook until slightly softened, about 4-5 minutes. Add garlic and cook just until fragrant.
Increase heat to medium high and add wine if using. Cook until almost evaporated, about 2-3 minutes.
Add mushrooms and soy sauce and cook until mushrooms are cooked through and tender, 4-5 minutes more.
Stir in parsley and serve.
