Heat olive oil in a large fry pan over high heat.

Sauté beef strips until browned but still pinkish for 1 minute per side. Transfer to a bowl.

Melt butter in the same pan; add leeks and sauté until soft, about 5 minutes.

Add sliced mushrooms and sauté for another 5 minutes. Season with salt and pepper.

Stir in tomato paste and cook until blended, about 1 minute.

Sprinkle flour over the vegetables and stir to coat.

Increase heat, add beef stock and bring to a boil for 1 minute, then reduce to medium.

Add creme fraiche, Dijon mustard, and lemon juice; cook for 1 minute. Adjust seasoning.

Return the beef to the pan and cook until heated through, about 2 minutes.

Serve over cooked egg noodles or pasta, garnished with finely chopped fresh parsley.