rectangular pan (about 13x9 inches)
In many supermarkets, you can buy mushrooms that have not been prepackaged. Just buy as many as you need.
Remove the pan of vegetables from the oven when it's time to turn them over. Place pan on a heatproof surface such as the burners of your range, and close the oven door to retain the heat.
Cut the onion half into 4 wedges, then separate into pieces.