It’s Sliced Cheese on Bread. And It’s Rocking Copenhagen’s World.

Copenhagen is famed for its innovative gastronomy and elaborate dishes. But these days its hottest food is its most basic: just bread, butter and hard cheese. And Copenhagen’s top bakeries are in a dead heat to make the best one around. It’s called a bolle med ost (bun with cheese), shortened by many to BMO, an abbreviation that was entered into the biggest Danish dictionary this year. Often served open-faced, this sort-of-sandwich is all about the fundamentals — sparer than smorrebrod and a test of how sublime the simple can be. That’s a 180 from the best-known Danish cuisine, which...