1. Start by heating the olive oil in a large skillet over medium-high heat. Once hot, add the sliced beef sirloin. Cook until browned on all sides, about 4-5 minutes. Remove from the skillet and set aside.
2. In the same skillet, add the chopped onion and cook until translucent, roughly 3 minutes. Stir in the garlic and cook for an additional minute.
3. Add the sliced mushrooms to the skillet, cooking until they are browned and have released their moisture, approximately 5 minutes.
4. Stir in the Dijon mustard and Worcestershire sauce, allowing them to blend with the mushroom mixture for about a minute.
5. Pour in the beef stock, bringing the mixture to a gentle simmer. Let it cook for about 5 minutes, allowing the flavors to meld.
6. Reduce the heat to low, and stir in the sour cream. Return the beef to the skillet, and let everything simmer for another 5 minutes, stirring occasionally.
7. Season with salt and pepper to taste. Add a tablespoon of butter if you prefer a richer sauce.
8. Garnish with freshly chopped parsley before serving.

Remember, cooking is about finding joy in the process, not just the outcome. Allow yourself to make a mess and find your rhythm in the kitchen. It’s what makes every dish uniquely yours.