(this started life as a comment on Captain Awkward, but may have gone to moderation because I included a link. However, I felt I should keep a copy here too...)
- Ask for both dietary requirements/restrictions and preferences. You can't always please everybody, but asking for preferences as well increases the odds that you'll be able to delight them on purpose.
- Asking for preferences does mean that the guy who likes to eat nearly an entire cow as rare as possible will tell you about it. You will also hear from people who wouldn't have otherwise felt comfortable sharing their strong aversions, non-life-threatening medical issues, and other awkward food stuff. Those are the people you're asking this question to make life easier for.
- Take people at their word. Don't badger them for details. If you need to ask clarifying questions, be respectful. "Can't you just pick them out" is not respectful.
- Don't take any requirement, restriction, or preference personally.
- Ask for dietary requirements/preferences ahead of time and keep them on file, so if there's a last-minute thing you can order with reasonable confidence.
- Ask for dietary requirements/preferences fairly shortly before finalizing each order, with a deadline, so people know when they have to get their information to you.
- Cross-check and update your file every time someone gives you new information. Stuff changes all the time -- new diagnosis, new medicine interaction, change in tastes, burned out on that thing after having it for 5 straight years in grad school.
- Treat the dietary requirements/preferences as somewhat confidential information unless you learn otherwise.
- Prepare a high-level summary of your team's dietary needs that you can give to catering. This protects your team's privacy (the catering crew doesn't generally need to know by name who goes with what preference) and makes things easier on the caterers who honestly probably don't care as long as nobody's getting something they shouldn't have. For example: group of 40, 30% vegetarian, 3 gluten-free omnivores, 1 lactose-intolerant omnivore, 2 shellfish allergies, 1 tree nut allergy (peanuts ok).
- Not everyone is going to want to eat the same thing. That's OK. Not everyone has to eat the same thing! If the group is large enough, you can order smaller amounts of a larger variety of things.
- Be on the alert for intersecting requirements. If the same person is dairy intolerant and gluten intolerant, it does them no good if all the gluten-free dishes have dairy and all the dairy-free dishes have gluten. The plight of the mushroom-allergic vegan is a sad one. I address this with a spreadsheet and a lot of swearing.
- Be on the alert for requirements that can be collapsed together. The main dish which will serve the uncomplicated vegan will also serve the person with lactose intolerance who can't have pork. I also address this with the spreadsheet.
- My spreadsheet is a matrix of people going down, and dietary restrictions going across. I like conditional formatting that highlights red and green to help me visualize. I also have a separate text document with their original phrasing because sometimes that's important. (Sometimes this is when I find I need to ask clarifying questions, such as if someone said "vegan, nut allergy" and the caterer is proposing coconut in the vegan stir-fry, does that nut allergy include coconut?)
- I assume that at least another 10% of people will like the look of the vegetarian entree than who are vegetarian as a restriction. When my group is 20% vegetarian I order for 30% vegetarian.
- Throw yourself on the caterer's mercy if that's a practical option. I tend to give them the restrictions summary and order "chef's special" which results in cost-effective, seasonal dishes which meet the requirements and surprise my team who are used to cafeteria food (plus) although it's a surprise what they're going to be (minus).
- Demand ingredients labels as a matter of course, with special attention for the dietary requirements in your summary. The mashed potatoes might be: Potatoes, olive oil, salt. VEGAN, GLUTEN-FREE
- My workplace cafeteria is so fucking sick of hearing "the stir fry has bell peppers and the menu doesn't say that it had bell peppers in it", but someday the fuckers will learn that they've got to list all their nightshades.
- Prepare a contingency plan for what to do if the catering falls through or somebody can't eat what's there.
- The best $REQUIREMENT-compliant dishes are not the ones that have a substandard substitution for the forbidden ingredient, but ones which were never intended to have any in the first place.
- I have been using
http://captainawkward.com/2013/05/15/479-trying-to-be-more-social-when-you-have-serious-dietary-restrictions/#comment-53962 as a guideline on diversifying the menu for a while.