Pat the meat dry and season liberally with salt and pepper.

Heat a large sauté pan over medium heat and melt 1 tablespoon of butter until it is gently sputtering. Add half of the meat and brown for about 1 minute, turn meat over and brown the other side, about a minute more. You want the meat to remain rare inside. Set aside in a bowl. Repeat with another 1 tablespoon butter and remaining meat. Add this cooked batch of meat to the first and set aside.

Heat 1 tablespoon of butter in pan now over low heat and add leeks and scallions, stirring often, and cook for a couple of minutes until softened, but not browned. Stir in mushrooms and continue to cook for a few more minutes, stirring occasionally, until mushrooms soften. Add 1 3/4 cups (420 ml) beef stock and the sherry. Turn heat up and simmer the mixture, stirring occasionally for about 3 minutes or until the liquid reduces a little bit, then turn off the burner. You can pause at this point to cook the pasta.

Meanwhile, bring a large pot of salted water to a boil and cook pasta until al dente. Drain and toss with last tablespoon of butter; set aside and keep warm.

Return to the stroganoff. Heat the mushroom sauce over medium heat, whisking in the mustard. Whisk the last 1/4 cup (60 ml) of stock with the cornstarch to make a slurry and add it to the hot mushroom sauce. Bring to a simmer and simmer gently for about 1 or 3 minutes until thickened and glossy. Turn heat down, whisk in sour cream, then add the meat and any juices that have exuded. Stir in 1 tablespoon dill and 1/4 teaspoon paprika. Cook just until meat is heated through; do not let the stroganoff oil.

Serve immediately over buttered noodles. Garnish with remaining 1 tablespoon dill and a sprinkling of paprika.