Prepare the Filling: In a small bowl, mix the dried spinach, softened cream cheese, minced garlic, and 1/2 cup of the mozzarella.
Prep Chicken: Cut a horizontal pocket into each chicken breast. Season inside and out with salt, pepper, and optional oregano.
Stuff: Divide the filling into 4 portions and stuff each breast. Secure the opening with toothpicks.
Sear: Heat olive oil in a large oven-safe skillet over medium-high heat. Sear chicken for 4-5 minutes per side until golden.
Bake: Top each chicken breast with Roma tomato slices and the remaining 1/2 cup of mozzarella.
Finish: Bake at 450°F (232°C) for 15 minutes. Optional: Broil for 2 minutes until the cheese is bubbly and browned.
Serve: Remove toothpicks and rest for 5 minutes before serving.