I'm surprised that people prefer under-cooked burgers as in the uk burgers cooked to under 160 degrees (Well done")are con sidered hazardous and it is considered a food risk to undercook burgers and in particular to hold them at lower than that.
holding is a whole different issue, not only with the beef but for instance sauteed onions & even salads/greens.
The places I go for a med rare burger are making it to order so no hold time.
Just like anyone who enjoys sushi or other raw/undercooked items, one has to have faith that the restaurant is using high quality vendors/supplies. I think my faith may be a
bit stronger than the usual consumer
