Shirred eggs elements
Only two elements are necessary for shirred eggs:


Fat to grease the baking dish, so that the eggs do not stick unduly
Eggs


I would suggest a touch of salt is the only one that is truly important, as it is in almost every dish.

The rest&mdash;cream, spinach, ham, cheese, and so on&mdash;are simply to add flavor.  They are only as important as your enjoyment of their flavor, or the pressure to use up leftovers before they go bad.  

For the dish at its most basic, see Julia Child's recipe at WGBH:  it consists of nothing but eggs, butter, salt and pepper.  Of course, it is immediately followed by suggested variations with garnishes.

I suspect you will never find a single canonical version of a dish this simple.  It is like asking "What is the classic version of an omelette?"  At its heart, an omelette requires little more than eggs, fat, and dairy&mdash;but more than this, it is a foundation on which generations of cooks have wrought their creativity, or just used up their leftovers. 

Shirred eggs, much the the same way, are a foundation on which flavors can be built, or leftovers expended.  The basic recipe, for both shirred eggs and omelettes, is the starting line, not the finish line.