1 or 2 finely-chopped mushrooms
Salt and pepper
Slices of ham
Having cooked and chopped or sieved the spinach, nicely seasoned with salt, pepper and butter or cream, add to it sufficient rich white sauce to make about two cupfuls in all. Take small ramekins, butter them well, place a little chopped parsley, a drop or two of onion juice and a teaspoonful of finely-chopped mushrooms, previously gently cooked in a little butter with salt in each ramekin and, on this, break an egg, seasoning with salt and pepper. Put the cases in a pan containing hot water, and cook gently in the oven until the eggs are set but not hard. Dish up the hot spinach in an entrée dish; on this set the croûtes of fried bread and carefully turn an egg on to each round. Serve with slices of mild ham as an entrée or supper dish.
Since the Encyclopaedia predates (1952) by eight years the good Doctor's wonderful book, I wonder if he was familiar with this recipe? The Encyclopaedia refers to it (p. 617 of the 1981 Overlook Press hardback edition) simply as "Green Eggs", but as you can see the ham is presented as an integral part of the recipe, not merely as a garnish.