Simple. If a customer asks about any allergen, the answer is “yes, everything on the menu is cooked in or exposed to peanut oil and may cause a reaction”, “yes, everything on the menu contains or is exposed to wheat and may cause a reaction”, and “yes, everything on the menu has been exposed to shellfish and may cause a reaction”. I would not want the risk of a customer with allergies who wants to use this law. Thinking back to the moron who sued McD’s because their fries had been exposed to beef (no kidding - everything in the restaurant is coated with a layer of oil from cooking fries and a layer of oil from the burgers).
That is exactly what I would do. Unfortunately, the nanny staters would probably make it an ADA requirement that I have to serve them something they can eat.