Also known as tiger meat or wildcat, cannibal sandwiches are a longtime Wisconsin tradition. Although the name implies something much more sinister, the sandwich typically consists of raw beef, spices, and a hefty helping of raw onions served open-face on rye bread or crackers. “We always had two kinds of bread, rye and white, since us kids were picky,” says Sutton.
-Brad_in_IL
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I saw these in bars every winter in Germany, but I don't think it was just a Christmas thing. It seems like I've seen them after the New Year, but don't take my word on that.
They were on rolls, not sliced bread, and they had thick slices of onion on top.
I may have even tried one.