ALFTA, Sweden—Lisa Englund probably wasn't thinking straight when she agreed to give Scandinavia's most notorious culinary classic a try.

"I've made a promise to my grandmother that I would eat at least one fish," the 21-year-old law-school student said while holding a cracker loaded up with onions, potatoes and pinkish-brown meat. She had just taken the first bite. "I wish I could say I like it, but I really don't."

Sitting...