Look in the frozen-food case for bags of frozen chopped onions. Keep them on hand for fuss-free chopped onion.
American cheese can be purchased in bulk at most supermarket delis. Simply ask them to cut off the size chunk you desire, and either chop or shred it before adding to the milk mixture.
To help with last-minute details on Thanksgiving Day, have the cauliflower washed and cut into florets, chop the onion and have the cheese cut into pieces. You can even mix the bread crumbs and butter together and store covered in a small bowl.