To make the dressing, combine sugar, flour, mustard and the 3/4 tsp salt in a small saucepan. Add the egg yolks and milk. Cook and stir over low heat until thickened. (Do not boil.)
Blend in vinegar and butter. Cool.
Whip cream to soft peaks. Fold into the cooked mixture.
Meanwhile, in a covered stock pot, boil potates for 25-30 minutes, or until tender. Drain.
Peel and chop potatoes.
In a large bowl, combine potatoes, celery, green onion, relish, 1 1/2 tsp salt and pepper.
Stir in the cooked dressing, tossing lightly to coat vegetables.
Cover and chill.
Before serving, fold in hard boiled eggs.
Garnish with hard-boiled egg slices and fresh parsley, if desired.
