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Cook pasta: Bring a large pot of salted water to a boil. (Meanwhile, continue to next steps.) Cook pasta according to package directions. Drain and return to pot. Add 2 tablespoons butter and cover to let butter melt, then toss so butter coats noodles. Set aside until sauce is done.
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Make sauce: Melt 2 tablespoons butter in a large skillet over medium heat. Add onion and a little bit of salt and pepper and cook, stirring often, for 5 minutes. Add mushrooms and a little more salt and pepper. Cook, stirring frequently, until mushrooms begin to release their juices, about 8 minutes. (During this time, skip to next step and begin cooking steak.)
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Cook steak: Pat steaks dry with paper towels and season with salt and pepper. Heat remaining 2 tablespoons butter in a skillet over medium-high heat. Add steaks and cook for 3 minutes on each side, without moving. Turn heat down to medium and cook for 2 more minutes on each side for medium-rare. Remove steaks to a plate and tent loosely with aluminum foil. Let rest 10 minutes.
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Finish sauce: Add wine to skillet with onions and mushrooms and cook for 1 minute. Add broth and soy sauce. Bring to a simmer and simmer for 5 minutes. Add cornstarch slurry and stir well. Cook 3-4 more minutes, until sauce is thickened. Remove from heat.
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When the steak is done resting and the sauce has cooled down a little, stir the sour cream into the sauce, just before serving. Taste sauce and add salt and pepper if needed. Slice steak against the grain. Divide noodles between serving bowls or plates and top with a generous amount of sauce and sliced steak. (Pour the sauce over the steak for a more traditional stroganoff.)