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Preheat oven to 375°F.
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Spray a medium skillet with cooking spray.
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Heat over medium high heat.
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Add onion.
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Cook stirring occasionally, until slightly soft, about 3 minutes.
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Mix onion, 1/4 cup ketchup, 1 Tbsp mustard, mozzarella cheese, bread crumbs, Parmesan cheese, and egg in a large bowl.
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Add beef, mix lightly.
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Spray a loaf pan with cooking spray.
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Press beef mixture into pan.
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Combine remaining ketchup and mustard in a small bowl.
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Spread over top of meat loaf.
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Bake meat loaf until set and cooked through, about 55 minutes.
