Add diced onion to pan, and cook for about 5 minutes.
Add wine and thyme; cook 3 minutes or until reduced by half.
Add stock and Dijon mustard. Cook until thickened, stirring occationslly - then add sour cream and whisk until smooth.
Return the chicken to skillet, cover and simmer 2 minutes or until heated through.
Serve pan sauce over the chicken.