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Adjust rack to middle and top positions and preheat oven to 400 degrees. Bring a large pot of salted water to a boil. Wash and dry all produce. Halve, peel, and dice onion; mince a few pieces until you have 2 TBSP (3 TBSP for 4 servings). Halve mozzarella; dice one half into four equal-sized pieces (eight pieces for 4); tear remaining half into bite-size pieces.