01 - Preheat oven to 400°F. Lightly grease a large baking dish with 1 tablespoon of the olive oil, coating the bottom and sides evenly.
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 the remaining 1 tablespoon of olive oil, then sprinkle with half the kosher salt, black pepper, Italian seasoning, and smoked paprika. Toss the vegetables gently until evenly coated.
03 - Pat the chicken breasts dry with paper towels. Season both sides of each breast with the remaining kosher salt, black pepper, Italian seasoning, and smoked paprika, pressing the spices gently into the meat. Lay the seasoned chicken breasts on top of the vegetable bed in a single even layer.
04 - Scatter the minced garlic evenly over the chicken and vegetables. Cover the baking dish tightly with aluminum foil and bake for 20 minutes, allowing the chicken to cook through gently in the steam.
05 - Remove the dish from the oven and carefully take off the foil. Sprinkle the shredded mozzarella and grated Parmesan evenly over each chicken breast. Return the uncovered dish to the oven and bake for an additional 10 minutes, or until the chicken registers 165°F internally and the cheese is fully melted with golden-brown spots.
06 - Remove the baking dish from the oven and let the chicken rest for 5 minutes to allow the juices to redistribute. Serve each breast over a portion of the roasted peppers and onions, spooning any pan juices over the top.