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1
Preheat a large skillet over medium heat for 3 to 4 minutes.
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2
Raise the heat to high and heat 1 tablespoon oil.
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3
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.
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4
(It is key to only partially cook the meat at this stage, since it will be finish cooking later in the sauce.)
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5
Transfer to a large plate and set aside.
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6
Repeat with the remaining oil and beef.
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7
Discard any excess oil.
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8
Return the skillet to medium-high heat.
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9
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.
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10
Season with salt and pepper to taste.
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11
Using a slotted spoon, transfer to the plate with the beef.
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12
Heat 4 tablespoons butter and, just as the foaming begins to subside, add the onion and cook, stirring, until lightly caramelized, about 5 minutes.
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13
Add the tomato paste and cook, stirring, until lightly browned, about 1 minute more.
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14
Whisk in the flour and cook, stirring, for 1 minute.
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15
Pour in the beef broth and, whisking constantly, bring to a full boil.
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16
Remove from the heat.
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17
Whisk in the sour cream, mustard, and lemon juice and season with pepper to taste.
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18
Set the sauce aside covered.
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19
Bring a large pot of water to a boil, salt generously, and cook the noodles until tender but not mushy.
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20
Drain and transfer to a large bowl.
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21
Toss with the butter and season with pepper to taste.
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22
Add the beef and any juices, mushrooms, and parsley to the sauce and reheat over medium heat until just hot.
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23
(Do not boil.)
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24
Divide the noodles among 4 plates and top with the stroganoff.
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25
Serve immediately.
