Cook the noodles according to package directions. Drain, transfer to a large bowl, and keep warm.
Meanwhile, whisk together the broth and cornstarch in a small bowl and set aside.
Heat the oil in a 12-inch nonstick skillet over medium-high heat. Add the mushrooms, onion, and garlic and cook, stirring often, until the liquid is evaporated and the mushrooms are lightly browned, about 8 minutes.
Stir in the beef, paprika, salt, and pepper. Re-whisk the broth mixture and add to the skillet. Cook, stirring constantly, until the sauce comes to a boil and thickens and the beef is heated through, about 2 minutes.
Add the parsley and butter to the noodles and toss to combine. Divide the noodles and beef mixture evenly among 4 plates. Top each serving with 1 tablespoon sour cream.