Heat the skillet. Set a large oven-safe skillet over medium heat.
Add 1 tablespoon olive oil. When it shimmers, add the onion and a pinch of salt. Cook 3–4 minutes until softened.
Add garlic and seasonings. Stir in garlic, Italian seasoning, and red pepper flakes.
Cook 30 seconds until fragrant, stirring so the garlic doesn’t brown.
Brown the chicken. Push the onions to the edges. Add remaining 1 tablespoon olive oil and the ground chicken to the center. Sprinkle with salt and black pepper.
Cook 5–7 minutes, breaking it up with a spoon until no pink remains and you get a few golden bits for flavor.
Build the sauce. Stir in tomato paste (if using) and cook 1 minute. Pour in marinara and chicken broth. Scrape up any browned bits from the pan.
Reduce heat to medium-low and simmer 5 minutes to thicken slightly.
Add Parmesan. Stir in half of the Parmesan until it melts into the sauce. Taste and adjust salt and pepper as needed. The mixture should be saucy but not watery.
Top with cheese. Sprinkle the mozzarella evenly over the skillet, then top with remaining Parmesan.
Cover with a lid and cook 2–3 minutes until the cheese melts. If your skillet is oven-safe, you can also broil 1–2 minutes for golden spots. Watch closely.
Finish and serve. Remove from heat.
Garnish with torn basil. Serve over pasta, spoon onto toasted bread, or keep it lighter with sautéed greens or zucchini noodles.