My "lunch advice" is to eat before you get on the ship. Once you do board, you will need to go to somewhere (the MDR is where we had to go) to meet with the dining team and go over my food allergy information that we submitted in advance. By the time we waited to do this, got our special tickets for the meet and greets, etc. our room was ready. We went there to drop off our bags. Then it was time for the muster drill and sailaway party.
My experience (and this is every cruise line, not just
DCL) the staff in the buffets are SO busy and overwhelmed for embarkation day lunch that it takes FOREVER to get the right person in front of you to walk you through what is "safe" and/or to have the kitchen make something special/safe. We've had everything from an hour plus wait (just standing awkwardly near the door to the kitchen) to "the only thing I can get for you right now is a hot dog without a bun and fries". Its not worth the stress of trying to get a lunch on board. Go, do your thing and by dinner everything will have settled. At breakfast and lunch the next day, things will be calmer.
Remember, half the dining crew is busy getting rid of all the stored food waste and inventorying the new food and beverage that was just loaded on for the new passengers. That all has to get stowed in the proper locations ASAP...which is why there are so few people available for embarkation lunch to assist.