Bring a large pot of salted water to a boil over high heat.
Season the beef with 1 tsp. salt and ¾ tsp. pepper. Heat 2 tsp. of the oil in a 12” skillet cook over medium-high heat until very hot. Add ½ of the beef and cook, stirring often, until the meat browns - about 1 minute. Do not overcook. Transfer to a plate and repeat with 2 tsp. of the oil and the remaining beef.
Remove skillet from heat. Add 1 tbsp. of butter and the remaining 2 tsp. oil to the skillet. Stir until butter melts. Put the skillet over medium heat and add the mushrooms. Cook, stirring until the liquid has evaporated and the onions begin to soften, about 3 minutes more.
Sprinkle the mushroom mixture with the flour and stir approximately 15 seconds. Stir in broth and bring to a simmer, scraping up any browned bits in the skillet with a wooden spoon.
Add the sour cream and whisk until sauce is smooth. Add the beef and any juices from the plate to the skillet. Cook, stirring often, until the sauce is barely simmering. Season to taste with salt and pepper.
Meanwhile, cook the noodles according to the package directions, until barely tender. Drain and return to the pot. Off the heat, add the remaining 2 tbsp. butter and stir to melt.
Serve the beef stroganoff over the noodles. Sprinkle with chives. A masterpiece!