Preheat and prep: Set your oven to 425°F (220°C). If your skillet isn’t oven-safe, have a 9x13-inch baking dish ready.
Sauté aromatics: Heat olive oil in a large oven-safe skillet over medium heat.
Add onion (and bell pepper, if using). Cook 4–5 minutes until softened. Stir in garlic and cook 30 seconds, until fragrant.
Brown the chicken: Add ground chicken, salt, and black pepper.
Break it up with a spoon and cook 5–7 minutes until no longer pink. Don’t worry about deep browning; just cook it through and keep it crumbly.
Season and build flavor: Sprinkle in oregano, dried basil, and crushed red pepper flakes. Stir in tomato paste and cook 1 minute to caramelize slightly.
Add tomatoes and simmer: Pour in crushed tomatoes.
If the sauce feels very thick, add up to 1/2 cup broth or water to loosen. Stir in sugar or balsamic to balance. Simmer 5–8 minutes, stirring occasionally, to marry the flavors.
Adjust and add veggies: Taste and season with more salt or pepper if needed.
If using spinach, mushrooms, or zucchini, fold them in now. Cook 2–3 minutes until just softened.
Top with cheese: Remove from heat. Sprinkle mozzarella evenly over the surface, then dust with Parmesan for extra savory bite.
Bake: Transfer the skillet to the oven.
Bake 10–12 minutes, until the cheese is melted and bubbling at the edges.
Broil for golden top (optional): Switch to broil for 1–2 minutes to get beautiful browned spots. Watch closely—cheese goes from golden to burnt fast.
Rest and serve: Let the bake rest 5 minutes. Garnish with fresh basil or parsley.
Serve over pasta, with crusty bread, on top of rice, or alongside a green salad.