For the dressing, Combine the egg, sugar, mustard and cornstarch in a medium saucepan. Whisk until smooth. Add the vinegar and water and cook until thickened. Remove from the heat. Stir in the mayo, sour cream and butter. Set aside to cool.
Place in a large bowl and toss broccoli florets, cashews, swiss cheese, oranges, cranberries, bacon and onion. Just before serving, pour cooled dressing over salad and toss to coat.