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Preheat oven to 400 degrees.
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In a medium bowl, toss together chicken cubes, sea salt and black pepper.
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Heat oil in a large skillet and sauté onion and garlic until fragrant. Add chicken and sauté until just cooked through. Add marinara sauce, oregano and fresh basil; heat through.
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Meanwhile, cook pasta according to package directions; drain and return to pot; add chicken and sauce, along with mozzarella cheese; mix well. Divide between two 8-inch square dishes or pour the entire pot into a 13x9x2 inch casserole dish.
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In a small bowl, mix together bread crumbs and Parmesan cheese. Sprinkle on top of casserole dishes. Place the pieces of butter on top. Bake for 15 minutes; then broil for a 2-3 minutes to brown the top.