Preheat oven to 375 degrees.
Place your chicken breasts of a flat surface. Using a sharp knife, cut a long pocket in each chicken breast, making sure not to slice all the way through the chicken.
Spoon about 1 1/2 Tbsps of sun-dried tomato pesto inside of each chicken breast. Place about 7-8 large spinach leaves and 2 Tbsp shredded cheese inside of each chicken breast.
Secure each chicken breast with toothpicks (I used 3 in each just to be safe).
Mix black pepper, salt, dried Italian seasoning, garlic powder and fresh rosemary together. Sprinkle the mixture evenly over each chicken breast.
Add oil to a large skillet (I used a cast iron skillet, but feel free to use any oven safe skillet). Sear each side of the chicken breast for about 1-2 minutes, or until lightly golden brown.
Turn off the heat.
Mix milk, chicken stock, garlic powder, salt and pepper together. Pour the mixture into the skillet, around the chicken. Add fresh spinach around each chicken breast. I used 1 cup of fresh spinach, but you can use more or less depending upon your preference.
Bake in the oven for about 18-20 minutes, or until chicken is cooked thoroughly (at 165 degrees Fahrenheit). Sprinkle with fresh rosemary, if desired.
Serve immediately.