Heat 1 teaspoon water in a skillet over medium high heat. Cook the diced apple until tender, about 3 to 4 minutes.
In a small bowl, mix the cooked apple, cranberries, shallots, feta and a splash of the chicken stock and 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. Place 1/4 of the apple mixture in the pocket of each chicken breast. Secure the pocket with toothpicks threading along the side to close.
Heat the remaing 1 1/2 tablespoons oil in a heavy oven-proof skillet until it begins to smoke. Cook each side of the chicken until golden brown.
Add the vinegar and chicken stock and bring to a boil. Lower the heat and gently simmer the chicken for 2 or 3 minutes per side until cooked through.
Remove the chicken breasts from the skillet and keep warm. Continue to cook the sauce until it is reduced to a thick syrup.
Taste the sauce and season with salt and pepper. Spoon the sauce over each chicken breast to serve.