1½ tbsp. olive oil, divided
¾ lb. top sirloin, cut into strips 1-inch wide and 2-inches long
Salt and pepper, to taste
1½ tbsp. butter
2 leeks, white and light green portions only, cleaned and finely chopped
8 oz. baby bella mushrooms, sliced
2 tsp. tomato paste
1 tbsp. all-purpose flour
1¼ cups beef stock
2-3 spoonfuls sour cream
1 tsp. Dijon mustard
1 tsp. lemon juice
Cooked egg noodles, for serving
Fresh parsley, finely chopped for garnish
In a large skillet over medium-high heat, warm the about half of the olive oil. Pat the beef dry with paper towels and season with salt and pepper. Add some of the beef strips in a single layer, being sure not to crowd the pan. Sauté until browned but still slightly pink on both sides, about 1 minute per side. Transfer to a bowl. Repeat with remaining oil and meat.
In the same pan over medium heat, melt the butter. Add the leeks and sauté until lightly browned and tender, about 5 minutes. Add the mushrooms and sauté until nicely browned, about 5 minutes more. Season with salt and pepper to taste.
Stir in the tomato paste and cook until well-blended, 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. Boil for 1 minute, then reduce the heat to medium. Return the meat to the pan along with any accumulated juices and cook just until the beef is heated through, about 2 minutes. Stir in the sour cream, mustard and lemon juice and heat just until the sauce is warm. Remove from the heat, serve over egg noodles and garnish with fresh parsley.