If you're using frozen broccoli, be sure to rinse off any ice or frost and dry the broccoli well. You don’t need to thaw it first, but if you do, make sure to pat away any excess moisture before adding it to the casserole. Make sure to drain the broccoli well after cooking, whether using fresh or frozen. Also, let the casserole rest for 10 minutes after baking to allow everything to set and thicken.