Preheat the oven to 375°F (190°C).
Place each chicken breast between two sheets of plastic wrap and gently pound with a meat mallet or rolling pin until about 1/4 inch thick.
In a medium bowl, combine the chopped spinach, cream cheese, shredded mozzarella, grated Parmesan, minced garlic, dried Italian seasoning, salt, and black pepper. Mix well until fully combined.
Spread an even layer of the cheese and spinach mixture over each flattened chicken breast.
Carefully roll up each chicken breast tightly, starting from one end, and secure with toothpicks to hold the shape.
Heat the olive oil in an oven-safe skillet over medium-high heat.
Add the chicken roll ups and sear on all sides until golden brown, about 2-3 minutes per side.
Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C).
Remove the toothpicks before serving. Let the roll ups rest for 5 minutes, then slice and serve warm.