We have large family gatherings and one oven. Many of us put several things (casseroles, etc.) in the oven at the same time. It seems to take things longer to cook this way. Is there any "rule of thumb" for increasing the cooking time when placing more than one item (either appetizers or casseroles) in the oven if the temperature required for all items is the same?

 Yes, the more cold stuff you put in the oven, the longer it takes to get it all hot. Unfortunately, the amount of additional time depends on how much cold stuff you put in, how dense each item is, how well your oven heats, what temperature the food is when it goes into the oven, and a few other variables. There is no rule of thumb.

Depending on how thick or dense or big the items are that you're cooking together (and how many there are), we would add 15% to the cooking time before we started checking to see if they were done. It might take 25% or even 50% more time, though — it’s just something you'll have to find out for yourself, and the result will be different the next time when you're cooking different foods. (You may also notice that with so many cooks putting things in and taking things out, your oven is losing a fair amount of heat, and that only adds to the time it takes everything to cook.)

If you have the resources and a little extra space, you might consider purchasing a large countertop oven or a roaster oven, in which to cook the appetizers, casseroles, or main course. Even if you just take it out of storage once or twice a year, it can make a big difference in the efficiency and scheduling of your family feasts.

Do be alert, though. The worst and/or cheapest of these appliances may burn your food, throw off unacceptable amounts of heat, etc. Many roaster ovens need to be heated for an hour or two before you cook with them (perhaps in a garage) to burn off any oils from the manufacturing process. Also, experiment with them a little; get to know your new appliance before feast day, so you don't have unexpected surprises.

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