Preheat the oven to 400 degrees F.
Bring a medium pot of salted water to a boil over high heat. Add the pasta and cook until just tender, stirring occasionally, about 5 minutes. Drain pasta into a large mixing bowl.
Meanwhile, put the olive oil in a medium saute pan over medium heat. Add the chicken and cook for 3 minutes. Add the onions and garlic, stirring to combine, and cook until the onions are soft and the chicken is cooked through, about 5 minutes more. Put the chicken mixture into the bowl with the cooked pasta. Add the canned tomatoes, mozzarella cheese, parsley, salt, and pepper. Stir to combine. Place the mixture in a buttered 8 by 8 by 2-inch baking dish. In a small bowl mix together the bread crumbs and the Parmesan cheese. Sprinkle over the top of the pasta mixture. Dot the top with small bits of butter. Bake until the top is golden brown, about 30 minutes.
Recipe courtesy of Giada De Laurentiis