Cook onion in butter in large skillet until soft.

Add remaining ingredients and cook just until cheese is melted.

Put in 2 quart casserole dish.

Bake at 350 (uncovered) for 1 hour.

To make ahead and freeze: Mix all ingredients together (start with your rice cooked, not raw) and spoon into two or even three smaller casserole dishes, cover, label and freeze. If you thaw overnight, just cook the one hour. If cooking from frozen state, add ten minutes or so, check for doneness.