Heat 2 teaspoons olive oil in a large skillet over medium heat.
Add the 1 1/4 cup onion (diced), and cook until translucent.
Add the 4 garlic cloves (crushed) and cook 1 minute.
Add 1 1/2 cup fresh baby spinach, and cook 3 minutes (until wilted).
Combine spinach mixture, 2 tablespoons crumbled blue cheese and 1 teaspoon Dijon mustard in a small bowl.
STUFF THE CHICKEN
Season 4 boneless skinless chicken breasts with salt and pepper
Cut a pocket in each chicken breast and stuff spinach mixture into each.
COOK THE CHICKEN
Heat 1 tablespoon olive 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 1 cup onion to pan, and cook for about 5 minutes.
Add 1/3 cup dry white wine and 1 teaspoon dried thyme; cook 3 minutes or until reduced by half.
Add 1 cup chicken stock and 2 tablespoons Dijon mustard. Cook until thickened, stirring occasionally - then add 1/4 cup sour cream and whisk until smooth.
Return the chicken to skillet, cover and simmer 2 minutes or until heated through.