How do I add flavor to this amazing looking dish?
These look like a recipe I've seen going around recently called cloud eggs, with the yolk nesting in well-beaten whites like it's sitting in a cloud. (I'm only including that name because it may be easier for you to Google recipes with a more modern name than your pre-WWII version, in case you're looking for flavoring inspiration.)

Whether a sweet or savory meringue, the key to adding any seasoning is very, very gentle folding to avoid deflating the beaten egg whites. However, you should be able to add salt, spices, and I've even seen recipes that call for grated Parmesan, as long as you fold carefully instead of stirring vigorously.