Konnie.. I've watched you ask this same question several times now. And I have mixed reactions, so I've kept my trap shut. It can be done, and is done alot at many high volume concession stands (zoos, parks, amusement parks).. And if your going to do it, don't look for any praise or possibly risking return customers. We had a discussion here a year or so about this.. When I worked for a national concession company, we would pre cook the burgers prior to the events end (always knew when we would get slammed). We held them in a hotel pan on the steam table, with some water added, then the entire pan wrapped in foil. Soon as we knew the rush would come, we did the cheese and condiments, wrapped them in wax paper and put them on a heating pad. Needless to say, it worked.. Food was cooked, bun and toppings where somewhat fresh, but that meat still had a 'hold' flavor to it. Someone in the discussion experimented using some beef base with the water and reported good results at the time.
IMHO, if you need a quick turnaround, and avoiding the mess of holding, make sliders... use 1/8lb patties pre flattened then toss on the grill when they order. These will cook in a minute or so over an open flame (flip once). Customer then can order what level of burger they want (1/8, 1/4, 1/3lb burger etc). You just quick cook the # of patties (to make up the weight) to order and stack them up on a bun.
One thing about doing the hold thing, your in an open area and your customers will see what your up to. Could risk selling nothing... If they see you doing everything fresh, word of mouth could help you greatly down the road.
Found the discussion on holding burgers:
http://www.roadfood.com/Forums/topic.asp?TOPIC_ID=16194 Another option:
http://www.roadfood.com/Forums/topic.asp?TOPIC_ID=6998