As L_Rahman said, typically you register your allergies with your reservation.

That said, it's entirely possible they're unable/unwilling to accommodate every allergy and/or preference. There are well regarded restaurants near me that only do prix fixe tasting menus that I don't dine at as I know based on what they tend to serve that the likelihood of me being served things I won't want is high, and at that price point it's not worth it to me.