Preheat your oven to 375°F (190°C).
Using a meat mallet or rolling pin, gently pound the chicken breasts to an even thickness of about ½ inch.
In a mixing bowl, combine chopped spinach, cream cheese, dried cranberries, mozzarella cheese, minced garlic, salt, black pepper, and thyme (if using). Mix until well combined.
Lay the chicken breasts flat on a cutting board. Place a generous spoonful of the spinach mixture in the center of each breast.
Carefully fold the sides of the chicken over the filling and secure it with toothpicks or kitchen twine.
Heat olive oil in a skillet over medium heat. Once hot, sear the stuffed chicken breasts for about 3-4 minutes on each side, or until golden brown.
Transfer the seared chicken breasts to a baking dish and bake in the preheated oven for 20-25 minutes or until the chicken is cooked through and reaches an internal temperature of 165°F (75°C).
Once cooked, remove the toothpicks or twine, let the chicken rest for a few minutes, then slice and serve.