Before you dive into cooking, it’s essential to have everything organized. Finely chop the onion, clean and quarter the mushrooms, and thinly slice the pickled cucumber. Don’t skip this step—it makes the cooking process smooth and efficient!

Now, take the beef tenderloin and cut it into strips about finger-width. This size ensures even cooking and allows the meat to soak up all those delicious flavors. Set these aside for later.

In a large skillet, melt the butter over medium heat. When the butter has melted and starts to foam, add the chopped onions. Cook them until they turn translucent—about 3 to 5 minutes. This step builds the base for your rich sauce.

Next, add the prepared beef strips to the pan. Stir them in to coat them with the butter and onions. Continue to cook them until they turn a nice white color—this shouldn’t take more than 5 minutes. Season this layer with salt and pepper to taste, enhancing the flavor of the meat.

Once the beef is looking lovely, toss in the quartered mushrooms. Cover the pan and let everything steam together for about 5 minutes. This will help the mushrooms release their moisture and infuse the meat with fantastic earthy flavors.

Now, uncover the pan and mix in the sliced pickled cucumber and the white wine. Bring this delightful medley to a boil and let it bubble for a couple of minutes. This brief simmer allows the alcohol to evaporate while keeping the sauce lovely and tangy.

Stir in the Dijon mustard and the sour cream, blending them into the sauce. Allow the dish to simmer for a few more moments until everything is combined and heated through. At this point, your kitchen should smell divine!