I begin by heating the oven to 375 °F (190 °C). While it warms up, I grease the bottom of my baking dish lightly.
In a bowl, I stir together ricotta, ½ cup mozzarella, ¼ cup Parmesan, egg, basil, salt, and pepper until everything is well mixed and creamy.
I cut a pocket into the side of each chicken breast (carefully, so I don’t slice all the way through). Then I spoon about 2 Tbsp marinara and a quarter of the cheese mixture into each pocket.
I place the stuffed breasts in the baking dish. I season with salt and pepper, sprinkle the Italian seasoning, and drizzle the olive oil on top.
The chicken goes in the oven for 25–35 minutes, until the internal temperature reaches ~155 °F (so it's nearly cooked through). I discard excess juices from the pan at this point.
I switch the oven to broil. I top each chicken breast with the remaining 1 cup marinara, ¾ cup mozzarella, and 2 Tbsp Parmesan. Then I broil 4–5 minutes until the cheese is melted, golden, and bubbly, and the internal temp hits 165 °F.
Finally, I remove the dish, let it rest slightly, and sprinkle extra fresh basil over the top before serving.