To make ahead and freeze, omit preheating and prepare through Step 6 before putting in the oven to bake. Cover the prepared au gratin dish with foil. Freeze up to 1 month.
To serve, thaw in the refrigerator 24 hours (casserole will still be icy). Preheat oven to 350°F. Bake, covered, about 1 hour or until edges are bubbly. Let stand for 5 minutes before serving. Sprinkle with parsley.