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.