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Preheat oven to 350°F.
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In large bowl, beat egg.
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Add ground beef, bread crumbs, and onion; mix well.
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On waxed paper, press meat mix into rectangle.
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Lay 2 slices cheese on top.
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Spread ketchup and mustard on cheese (don't get to close to edges it may leak out).
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Roll into a long loaf, sealing edges.
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Slice remaining piece of cheese diagonally and reserve.
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Place meatloaf into loaf pan (may need to "smoosh" sides to fit).
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Bake 50-60 minutes or until internal temp is 175°F (if your meat isn't lean you may need to drain some of the grease out of the pan).
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Top with remaining cheese and bake 5 minutes or until cheese is melted.