Preheat and prep the pan: Heat the oven to 400°F (200°C).
Lightly oil a baking dish or line a sheet pan with parchment for easy cleanup.
Make the cheesy filling: In a bowl, combine mozzarella, cream cheese, Parmesan, spinach, garlic, parsley, lemon zest, and red pepper flakes if using. Mix until smooth. Season with a small pinch of salt and pepper.
Butterfly the chicken: Place a hand on top of each breast and use a sharp knife to slice horizontally, stopping about 1/2 inch from the far edge to create a pocket.
Do not cut all the way through.
Season the chicken: In a small bowl, mix salt, pepper, smoked paprika, and garlic powder. Pat the chicken dry with paper towels, rub with olive oil, and coat all sides with the spice mix.
Stuff the pockets: Divide the cheese mixture among the chicken breasts, gently spooning it into each pocket. Do not overfill.
If needed, secure openings with toothpicks to keep the filling inside.
Arrange and bake: Place stuffed breasts in the baking dish. Bake for 20–25 minutes, or until the thickest part reaches 165°F (74°C) on an instant-read thermometer.
Optional broil: For extra color, broil for 1–2 minutes at the end. Watch closely to avoid burning.
Rest and serve: Let the chicken rest for 5 minutes.
Remove toothpicks, slice, and garnish with herbs or a squeeze of lemon.