What's the point of bringing a casserole to room temperature before cooking?
When baking a casserole that you've pre-assembled and stored in the refrigerator, remember to bring it to room temperature (usually about 30 minutes) before subjecting it to the heat of the oven.
If you stick a cold casserole in a hot oven, you risk drying out the top of the casserole before you've given the inside time to cook or heat through.
If you want your casserole to cook evenly, do yourself a favor and let it come to room temperature first. If for some reason you can't, then as a last resort, reduce the heat to allow it to cook more slowly without drying out.