Prep the chicken: Pat the chicken dry and season with salt, pepper, and half the Italian seasoning.
Sauté the aromatics: Heat 1 tablespoon olive oil in a large skillet over medium-high. Add onion and bell pepper.
Cook 3–4 minutes until starting to soften. Add mushrooms if using, and cook another 2 minutes. Transfer to a bowl.
Cook the chicken: Add the remaining tablespoon of oil to the skillet.
Add the chicken in a single layer. Cook 4–6 minutes, stirring once or twice, until browned and cooked through.
Add garlic and spice: Stir in garlic, remaining Italian seasoning, and red pepper flakes. Cook 30 seconds until fragrant.
Simmer with sauce: Return the peppers and onions to the skillet.
Pour in the marinara. Stir and simmer 2–3 minutes to blend flavors. Taste and adjust salt and pepper.
Finish with cheese: Reduce heat to low.
Sprinkle mozzarella and Parmesan over the top. Add pepperoni or sausage if using. Cover the skillet for 2–3 minutes until the cheese melts.
Alternatively, transfer to oven-safe bowls and broil 1–2 minutes for browned, bubbly cheese.
Garnish and serve: Top with fresh basil or parsley. Serve hot in bowls with a side salad or steamed veggies.