- Heat 1 teaspoon olive oil in a skillet over medium-high heat. Add apple and cook until tender, 3 to 4 minutes.
- In a small bowl, combine cooked apple, cranberries, shallot, and 1 tablespoon chicken stock. Set aside.
- Cut a deep horizontal pocket in the side of each chicken breast. Make the pocket as large as you can without piercing the top or bottom of the breast. Divide apple mixture evenly among chicken breasts, stuffing into each pocket. Secure pockets with toothpicks, threading along the side to close.
- Heat remaining 1½ tablespoons olive oil in a heavy skillet. Cook chicken, turning once, until golden brown. Add vinegar and remaining chicken stock, then bring to a boil. Lower heat and gently simmer chicken, turning once, 2 or 3 minutes per side.
- When chicken registers 165 degrees Fahrenheit or higher using an instant-read thermometer placed in the thickest part of the breast, remove from skillet and keep warm.
- Continue cooking sauce until reduced to a thick syrup. Season with salt and pepper to taste.
- Spoon sauce over chicken to serve.