Preheat the OvenPreheat your oven to 375°F (190°C).
Prepare the ChickenSlice each chicken breast horizontally to create a pocket—being careful not to cut all the way through. Season inside and outside with salt and pepper.
Make the FillingIn a mixing bowl, combine ricotta cheese, chopped spinach, Parmesan cheese, minced garlic, Italian seasoning, red pepper flakes (if using), and a pinch of salt and pepper. Stir until well mixed.
Stuff the ChickenFill each chicken pocket with the ricotta mixture. Use toothpicks to secure if needed to keep the filling inside.
Sear the ChickenHeat olive oil in an oven-safe skillet over medium heat. Add the stuffed chicken breasts and sear for 3–4 minutes on each side until golden brown.
Add Sauce and CheesePour marinara sauce over the chicken and sprinkle the mozzarella cheese evenly on top.
BakeTransfer the skillet to the oven and bake for 20–25 minutes, or until the internal temperature of the chicken reaches 165°F (74°C) and the cheese is melted and bubbly.
Rest and ServeLet the chicken rest for 5 minutes before serving. Serve with extra marinara sauce or fresh herbs, if desired.