Bring 5 quarts of salted water to a boil in a stockpot.
Combine all the ingredients except the penne and butter in a large mixing bowl. Mix well. Drop the penne into the boiling water and parboil for 4 minutes. Drain well in a colander and add to the ingredients in the mixing bowl, tossing to combine. Divide the pasta mixture among 6 shallow ceramic gratin dishes (1 1/2 to 2 cup capacity). Dot with the butter and bake until bubbly and brown on top, 7 to 10 minutes.
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