In a large skillet, heat the olive oil and butter over medium heat until melted and hot.
Add the chopped onion and sauté for about 3-4 minutes, until softened and translucent.
Stir in the minced garlic and thyme, cooking for an additional 1 minute until fragrant.
Add the sliced mushrooms to the skillet and cook for about 5-7 minutes, stirring occasionally, until they are browned and have released their moisture.
Pour in the white wine and let it simmer for 2-3 minutes, allowing the alcohol to evaporate.
Add the broth and soy sauce, bringing the mixture to a gentle boil. Reduce the heat and let it simmer for about 10 minutes, or until the sauce has thickened slightly.
If using, stir in the heavy cream and cook for an additional 2 minutes. Season with salt and pepper to taste.
Remove from heat and garnish with chopped fresh parsley before serving.