01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with olive oil or nonstick spray.
02 - Arrange the sliced red, yellow, and green bell peppers along with the red onion in an even layer across the bottom of the prepared baking dish. Drizzle with 1 tablespoon of the olive oil, sprinkle with half the kosher salt and black pepper, and toss to coat evenly.
03 - Place the chicken breasts on top of the bed of vegetables. Brush with the remaining 1 tablespoon of olive oil, rub the minced garlic over each breast, and season generously with the smoked paprika, dried Italian herbs, and the remaining salt and pepper.
04 - Scatter the shredded mozzarella and grated Parmesan cheeses evenly over the chicken breasts and surrounding vegetables, ensuring full coverage for a golden, bubbly finish.
05 - Bake uncovered on the center rack for 25 to 30 minutes, until the chicken reaches an internal temperature of 165°F and the cheese is melted, golden, and bubbling. If the cheese browns too quickly, loosely tent with aluminum foil during the last 5 minutes.
06 - Remove from the oven and let the dish rest for 5 minutes so the juices redistribute. Serve hot, garnished with fresh parsley or basil if desired. Pair with a crisp green salad or steamed rice.