Preheat your oven to 400°F (200°C).
Sauté minced garlic in olive oil, then add spinach until it wilts completely. Squeeze out excess water.
In a bowl, combine the spinach, roasted red peppers, mozzarella, oregano, red pepper flakes, salt, and pepper.
Butterfly each chicken breast and gently pound it to an even 1/2-inch thickness.
Spoon the filling onto one half of each chicken breast.
Fold the chicken over the filling and secure the edges with toothpicks or butcher's twine.
Brush the outside of the chicken with olive oil and season with salt and pepper.
Place the stuffed chicken on a baking sheet.
Roast for 20-25 minutes, or until the internal temperature reaches 165°F (74°C).
Let the chicken rest for 5-10 minutes before slicing and serving.