Bring salted water to a boil, add noodles, and cook 8 minutes until al dente; drain and set aside.
While noodles cook, cut chicken into bite-sized pieces and slice mushrooms; dice onion.
Heat 2 tablespoons oil in a large pot over medium-high heat, then add chicken and cook until no longer pink inside, about 5 minutes; remove and set aside.
In the same pot, cook mushrooms and onion until softened, about 4 minutes, then stir in chicken broth and Dijon mustard and bring to a gentle simmer.
Return chicken to pot, remove from heat, stir in sour cream, then fold in cooked noodles and serve immediately.