Step 1 - In a large pot of boiling water, cook noodles, stirring occasionally, until al dente according to package instructions. Drain and transfer to a large bowl. Add 2 Tbsp. butter and toss to combine.

Step 2 - Meanwhile, slice steak against the grain into 2"-thick pieces. In a small bowl, mix salt and 3 Tbsp. flour. In a large, shallow pan over medium-high heat, heat oil until shimmering. Test an edge of a piece of steak; if the sizzle is aggressive, it's ready to go. Dredge steaks in flour mixture, shaking off excess. Cook, turning once, until deep golden brown, about 1 minute per side. Transfer to a cutting board.

Step 3 - In same pan over medium-high heat, combine mushrooms, 1 Tbsp. butter, and 1/4 c. water. Cook, stirring occasionally and scraping bottom of pan, until mushrooms are golden brown, 8 to 10 minutes; season with salt and stir to combine. Using a slotted spoon, transfer mushrooms to a plate.

Step 4 - Reduce heat to low. Cook shallots, 3 Tbsp. water, and remaining 2 Tbsp. butter, stirring occasionally, until shallots are caramelized and taste sweet, 10 to 15 minutes.

Step 5 - Add garlic and cook, stirring, until fragrant, about 1 minute. Increase heat to medium-high. Add mustard, tomato paste, and Worcestershire and cook, stirring, until paste is brick red, about 1 minute. Sprinkle with remaining 1 Tbsp. flour and cook, stirring, until flour is toasted, about 1 minute. Pour in stock and wine and bring to a boil. Cook, stirring occasionally, until slightly reduced, about 5 minutes. Transfer 1/2 c. sauce to a small bowl; stir in sour cream.

Step 6 - Thinly slice meat against the grain into 1/2" strips. Return steak and mushrooms to pan and toss to combine. Add sour cream mixture and noodles to pan and cook, tossing constantly, until sauce thickens and coats noodles, about 3 minutes; season with salt.

Step 7 - Divide beef stroganoff among bowls or plates. Top with parsley.

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