700 g top sirloin, cut into thin strips
3 tablespoons unsalted butter
3 tablespoons olive oil
3 leeks, white and light green portions only, finely chopped
500 g mushrooms, thinly sliced
1 tablespoon tomato paste
2 tablespoons all-purpose flour
2 1/4 cups beef stock
1/3 cup sour cream
2 teaspoons Dijon mustard
2 teaspoons fresh lemon juice
Salt and freshly ground pepper, to taste
Fresh flat-leaf parsley for garnish, finely chopped
1. In a large fry pan over high heat, warm 1 tablespoon of olive oil. Dry the beef strips with paper towels and season with salt and pepper. Add half of the beef strips (do not overcrowd the pan) and sauté until nicely browned but still a little pinkish on both sides, about 1 minute per side. Transfer to a bowl. Repeat with 1 tablespoon of olive oil and the remaining meat.
2. In the same pan over medium heat, melt the butter with the remaining 1 tablespoon of olive oil. Add the leeks and sauté until softened and lightly browned, about 5 minutes. Add the mushrooms and sauté until nicely browned, about 5 minutes more. Season with salt and pepper.
3. Stir in the tomato paste and cook until blended in, about 1 minute. Sprinkle the flour over the vegetables and stir to incorporate. Increase the heat to high, add the stock and bring to a boil, stirring with a wooden spoon to scrape up the browned bits from the pan bottom. Boil for 1 minute, then reduce the heat to medium. Add the sour cream, mustard and lemon juice and cook for 1 minute more to allow the flavors to blend. Taste and adjust the seasonings.
4. Return the meat and any accumulated juices to the pan and cook just until the beef is heated through, about 2 minutes. Garnish with parsley and serve immediately.
5. Place dumplings on platter, browned side up. Serve immediately with soy vinegar dipping sauce.
Serve with wide egg noodles or rice while it is warm.