Preheat your oven to 375°F (190°C)
Prepare the chicken breasts by butterflying them - lay each breast flat on a cutting board and cut along the side to create a pocket without cutting through the other side
Pat the chicken dry with paper towels and season with salt and pepper
For the stuffing, combine bread cubes, chicken broth, beaten egg, garlic powder, onion flakes, dried thyme, and any optional fresh herbs in a bowl
Add Greek yogurt, chopped celery, diced apples (if using), and shredded cheese (if using) to the stuffing mixture. Mix gently until just combined
Let the stuffing mixture sit for 10 minutes to allow flavors to meld
Fill each chicken breast with the stuffing mixture, leaving about ½ inch at the top to prevent overflow
Secure the stuffed chicken using toothpicks, kitchen twine, or by placing seam-side down on the baking sheet
Brush the chicken with olive oil or melted butter and season with additional spices if desired
Place on a parchment-lined baking sheet and cover with foil
Bake for 30 minutes covered, then remove foil and continue baking for 20-30 minutes until the internal temperature reaches 165°F (74°C)
Let the chicken rest for 10 minutes before serving to allow juices to redistribute