Cook pasta according to package; drain and set aside.
Season chicken with salt and pepper. Sear in olive oil over medium-high heat until cooked through; remove.
Reduce heat, melt butter, sauté onion until translucent, then add mushrooms and garlic; cook until browned.
Stir in flour for 1 minute, then whisk in chicken broth until smooth and slightly thickened.
Return chicken, stir in sour cream and Dijon; warm gently without boiling. Adjust seasoning.
Toss sauce with pasta, sprinkle parsley, and serve immediately.