Cook noodles with tablespoon salt according to package. Reserve 1/2 cup water; drain.
Meanwhile, season chicken with 1/2 teaspoon salt and pepper. Heat oil in large skillet; cook strips 6 to 8 minutes to at least 165°F. Plate.
Add butter, mushrooms, and onion; cook 8 to 10 minutes until mushroom water evaporates. Stir flour 1 minute; gradually add broth, mustard, paprika, and remaining salt. Simmer 3 minutes.
Return chicken; remove from heat. Whisk 1/4 cup warm sauce into sour cream; stir back into skillet.
Fold in noodles; warm over very low 1 minute without boiling. Add reserved water if needed for spoonable sauce; serve promptly.