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1
Using a meat pounder, pound the beef to an even 1/2 inch thickness.
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2
Slicing with the grain of the meat, slice the meat into 2-inch wide strips.
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3
Slice each strip crosswise into 1/2 inch wide strips.
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4
Pat the beef dry with paper towels and season with salt and pepper.
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5
Heat 1 tablespoon of the oil in a 12-inch skillet over medium-high heat until just smoking.
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6
Brown half of the beef until well-browned on both sides, 6 to 8 minutes.
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7
Transfer to a medium bowl and repeat with 1 more tablespoon of the oil and the remaining beef.
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8
Add the remaining 2 tablespoons of oil to the skillet and return to medium-high heat until shimmering.
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9
Add the mushrooms, onion and 1/2 teaspoon of salt, and cook until the liquid from the mushrooms has evaporated, about 8 minutes.
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10
(If the pan begins to scorch, add the accumulated beef juices to the skillet.
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11
).
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12
Stir in the flour and tomato paste and cook, uncovered, over low heat until the beef is tender and the sauce is thickened, 30 to 35 minutes.
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13
Remove the skillet from the heat.
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14
Stir in a few tablespoons of the sauce into the sour cream to temper, then stir the sour cream mixture into the skillet.
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15
Stir in the lemon juice and parsley and season with salt and pepper to taste.
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16
Toss with the noodles and serve immediately.
