Heat the Pan: In a large skillet, heat the oil and butter over medium heat until the butter is melted and the mixture begins to bubble.

Caramelize the Onions: Add the thinly sliced onions to the skillet. Stir occasionally and cook for 10-15 minutes until the onions soften and turn golden brown. Be patient—low and slow cooking allows the onions to caramelize and develop their natural sweetness.

Cook the Mushrooms: Once the onions are caramelized, add the sliced mushrooms to the pan. Cook for 5-7 minutes, stirring frequently, until the mushrooms are tender and have released their moisture.

Add Marsala Wine: Pour the Marsala wine into the pan, stirring to deglaze the bottom and scrape up any browned bits. Allow the mixture to simmer for 3-4 minutes until the wine reduces by about half.

Add Thyme and Crème Fraîche: Stir in the fresh thyme leaves and crème fraîche, letting everything combine. Season with salt and pepper to taste. Cook for another 1-2 minutes until the mixture is creamy and well-incorporated.

Serve: Remove from heat and serve immediately. This dish pairs beautifully with grilled meats, pasta, or as a flavorful addition to sandwiches and burgers.