Season beef with salt and pepper. Preheat a large skillet over medium heat. Add half of the oil and raise heat to high. Add half of meat to the skillet in a single layer. Cook until browned on the outside and just cooked through, 2 to 3 minutes. Transfer meat and juices to a plate and set aside, covered. Repeat with remaining oil and beef.

Put onions and a drop more oil, if necessary, in the skillet. Cook until onions are golden and tender, about 7 minutes. Transfer to the plate with the reserved beef.

Discard oil and any burnt bits remaining in skillet. Add butter and melt over medium-high heat. Add mushrooms and turn the heat up to high. Cook, stirring occasionally, until well-browned, about 5 minutes. Add tomato paste and cook, stirring, about 1 minute. Stir in flour and cook for about 1 minute.

Pour in broth and whisk constantly until liquid comes to a boil. Reduce heat and simmer for 5 minutes, stirring occasionally. Remove skillet from heat and stir in sour cream, mustard and lemon juice. Season with salt and pepper. Return meat and onions to skillet and toss to coat well with sauce. Serve hot, over the egg noodles, garnished with parsley, if desired.