1. Preheat a large skillet over medium heat for 3 to 4 minutes. Raise the heat to high and heat 1 tablespoon oil. Season half the beef with salt and pepper, add to the skillet, arranging it in a single layer, and saute, without stirring, until well-browned and still pinkish inside, about 1 to 2 minutes. (It is key to only partially cook the meat at this stage, since it will be finish cooking later in the sauce.) Transfer to a large plate and set aside. Repeat with the remaining oil and beef. Discard any excess oil.
2. Return the skillet to medium-high heat. Melt 2 tablespoons of the butter and, when the foaming begins to subside, add the mushrooms and cook, stirring occasionally, until well-browned, about 7 minutes. Season with salt and pepper to taste. Using a slotted spoon, transfer to the plate with the beef.
3. Heat 4 tablespoons butter and, just as the foaming begins to subside, add the onion and cook, stirring, until lightly caramelized, about 5 minutes. Add the tomato paste and cook, stirring, until lightly browned, about 1 minute more. Whisk in the flour and cook, stirring, for 1 minute. Pour in the beef broth and, whisking constantly, bring to a full boil. Remove from the heat. Whisk in the sour cream, mustard, and lemon juice and season with pepper to taste. Set the sauce aside covered.
4. Bring a large pot of water to a boil, salt generously, and cook the noodles until tender but not mushy. Drain and transfer to a large bowl. Toss with the butter and season with pepper to taste.
5. Add the beef and any juices, mushrooms, and parsley to the sauce and reheat over medium heat until just hot. (Do not boil.) Divide the noodles among 4 plates and top with the stroganoff. Serve immediately.
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