Sometimes, cooking multiple items in the oven at once is necessary. But it means the cooking time needs to be increased. Take a few things into consideration when figuring out how much time to add: the density of the cooking vessels, the type of food being cooked and the efficiency of the oven.
Decide how many items you're going to cook at once. Work out if they all fit in the oven together and if the heat can still circulate around each dish. If you're cooking totally different dishes together, work out which one takes the longest—it should go in first.
Check the suggested cooking time. If you've doubled or tripled a cake recipe and are cooking it in a large tin, then add 15 percent to the suggested time. If you're making two casseroles or have doubled a recipe, add 20 percent to the recommended cooking time.
After the cooking time is up, check for doneness. If you're baking cake, insert a toothpick into the center; if batter sticks to the toothpick, put the cake straight back in. If you're cooking meat, take it out of the oven and check the internal temperature with a cooking thermometer. If you don't have a thermometer, stick a knife into the center and check the color of the liquid. Chicken juices should be totally clear, not pink. Red meat is okay to eat with pink juices if you like it rare; for well-done meat, the juices should run clear.
Put the items that require longer cooking back into the oven. Estimate another 10 percent of the suggested cooking time before checking the dishes again. Generally speaking, if you're cooking two or three items all the same, increasing the cooking time by 10 to 15 percent should be fine. But if you have more items, or if the items are very dense, you might need to increase cooking time up to 30 percent.
Be sure to check each dish individually. Food being cooked on the bottom of the oven may take longer than food on the top shelf, particularly if the oven is not fan assisted.