Place a medium saucepan over medium heat. Add butter and let it melt slowly. The butter should become smooth and slightly bubbly without turning brown or burnt during cooking.

Add chopped shallots and sliced mushrooms into the pan. Stir often while cooking for about seven minutes. The mushrooms should become soft, tender, and lightly golden with rich earthy aroma.

Mix minced garlic into the mushroom mixture and cook briefly. Sprinkle flour evenly over everything. Stir carefully until the flour disappears completely and forms a smooth thick base for the sauce.

Slowly add beef stock while whisking gently. This prevents lumps from forming inside the sauce. Let the mixture simmer slowly until slightly thickened and smooth in texture.

Pour heavy cream into the saucepan and stir slowly. Add truffle oil, Worcestershire sauce, salt, and black pepper. Simmer gently for five minutes until the sauce becomes creamy and rich.

Remove the saucepan from heat once the sauce looks silky and smooth. Sprinkle chopped parsley over the top. Serve warm over steak, chicken, roasted vegetables or mashed potatoes.