Heat olive oil and butter in a skillet over medium heat.
Add chopped onions and sauté until translucent, about 3-4 minutes.
Stir in minced garlic and cook for another minute until fragrant.
Add the sliced mushrooms to the skillet and cook until soft, about 5-7 minutes.
If using, pour in white wine and let it reduce for 2-3 minutes.
Stir in the heavy cream and soy sauce, mixing well.
Season with salt and pepper to taste.
Bring the sauce to a gentle simmer and cook for another 5 minutes.
Remove from heat and let it cool slightly.
Garnish with fresh parsley before serving.