A law requiring the disclosure of, as you put it, the most common allergens (9) on menus wouldn't be hard to implement, and it wouldn't place a huge burden on restaurants.

It wouldn't even make the menus too "busy" if it was done with the same kind of coding already done to, say, indicate spice levels on a Chinese restaurant menu.

I know chefs don't want to reveal their recipes, and I understand that.

Hrm.

A higher technological solution would be a QR code on the menu that downloads an app (small restaurants could club together) that allows you to enter the thing you are allergic to and it would either flag menu items that contain it or those that don't. I'm not sure which way would work best - I think I would prefer the latter. Maybe it could be customized?

This would work for ALL allergens, not just the most common ones. Niche for an app developer?