Heat the skillet over medium heat and add the butter until melted.

Add the sliced onions and sauté for about 5 minutes until they become translucent.

Stir in the minced garlic and cook for an additional minute until fragrant.

Add the sliced mushrooms to the skillet, season with salt and pepper, and cook for about 8-10 minutes until the mushrooms are tender and browned.

If using, add the soy sauce and stir well to combine.

Remove from heat and garnish with chopped parsley before serving.