Cut the beef into 1" chunks and set aside (or strips).
Start the water for the noodles. Spray a large nonstick skillet with cooking spray and cook the onions over medium-heat until tender, about 10 minutes. When the onions have been cooking for about 5 minutes, cook the noodles. Remove the onions from the skillet and set aside.
Combine the sour cream, flour, ketchup, Dijon mustard, salt, pepper, and 2 tablespoons of the chives in a small bowl and set aside.
Heat the olive oil in the same skillet and cook the beef and mushrooms over medium-high heat, stirring frequently until beef is slightly browned, about 5 minutes.
Reduce the heat to low and return the onions to the skillet. Add the sour cream mixture to the skillet; stir until well blended and slightly thick.
Serve over noodles and sprinkle with the remaining tablespoon of chives.