Combine spinach mixture, cheese and mustard in a small bowl.
STUFF THE CHICKEN
Season chicken with salt and pepper.
Cut a pocket in each chicken breast and stuff spinach mixture into each.
COOK THE CHICKEN
Heat the remaining oil in skillet over medium-high heat.
Add chicken and cook 6 minutes on each side or until chicken is done (165 F). Remove chicken, tent with foil to keep warm and set aside while you make the pan sauce.
PAN SAUCE
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.