Preheat the oven to 375°F (190°C).
In a small bowl, combine the shredded mozzarella and grated Parmesan; set aside.
Trim any excess fat from the chicken breasts and use a sharp knife to cut a pocket into the side of each breast, being careful not to cut through.
Season both sides of each chicken breast with Italian seasoning, garlic powder, salt, and pepper.
Stuff about 1/4 cup of the cheese mixture into each chicken pocket and fold closed; secure with toothpicks if desired.
Heat 1 tablespoon olive oil in an oven-safe skillet over medium-high heat. Sear the chicken breasts 3–4 minutes per side until golden brown.
Remove the skillet from heat, spoon marinara sauce over each chicken breast, and top each with a few tablespoons of the reserved cheese.
Bake uncovered at 375°F (190°C) for 15–20 minutes, or until an instant-read thermometer inserted into the thickest part of the chicken reads 165°F (74°C).
Let the chicken rest a few minutes, remove any toothpicks, and serve.