In a large skillet, heat the olive oil over medium-high heat. Add the beef strips and sear until browned on all sides. Remove the beef from the skillet and set aside.

In the same skillet, add the chopped onion and sauté until translucent, about 3-4 minutes. Add the minced garlic and cook for an additional minute until fragrant.

Add the sliced mushrooms to the skillet and sauté until they are golden brown and their moisture has evaporated, about 5-7 minutes.

Sprinkle the crushed black peppercorns over the mushrooms and stir to combine.

Reduce the heat to medium-low and pour in the heavy cream, stirring until it begins to thicken.

Add the Dijon mustard and Worcestershire sauce, mixing well. Season with salt to taste.

Return the seared beef strips to the skillet, stirring to coat them in the creamy mushroom sauce. Simmer for an additional 5 minutes, allowing the flavors to meld.

Serve hot, garnished with freshly chopped parsley, over rice, pasta, or mashed potatoes.