The New York Times reports that,

As French chefs have embraced the quintessentially American food, they have also made it their own, incorporating Gallic flourishes like cornichons, fleur de sel and fresh thyme. These attempts to translate the burger, or maybe even improve it, strongly suggest that it is here to stay.

The story has a lot of food porn about $50 burgers and nothing about food safety. Or thermometers.

Frédérick Grasser-Hermé, consulting chef at the Champs-Élysées boîte Black Calvados, said,

“A hamburger is the architecture of taste par excellence. The meat needs to be a mix of fatty and lean. Not raw, not rare. It must be medium rare. At the same time the bread needs to be smooth, tepid, toasted on the sesame side. I like to brush the soft side with butter. There needs to be a crispy chiffonade of iceberg lettuce. Everything plays a role.”

Rare, medium-rare, these terms are too subjective. Use a thermometer, and stick it in.