At the bar of a swanky new French restaurant, I spied a woman wearing a beret. “Amélie” was being projected onto a wall. I, having a French moment of my own, was eating escargot drenched in butter.

Ever since Maria Grazia Chiuri sent every single model down the runway topped with a leather beret in Dior’s AW17 show earlier this year, the long abandoned French millinery of choice has been ...